Precihole VX100: An Alternative State of Tune!

[prashantjha19]

Precihole’s air rifles, as of some other local manufacturers’, have evoked great response and appreciation among enthusiasts; their springers range being no different! And yet, having owned and used one for quite some time now, my efforts to verify some technical aspects and attributes on various forums over these past months hardly yielded anything concrete, apart from some purely subjective feedback. Either most of us are unsure, or lack the needed info, or are simply reluctant to share. None, however, bodes good for either the sport or our fraternity! I, for one, have grown up with the belief that airgunning (as a sport and hobby) needs free sharing as much as plentiful practice and, of course, lots of tinkering!

[Pls check the add-on post made on 26 June 2018 for some additional info and clarification...]

Shoots great out-of-box...but with perceptible surge!!
[PS: For more on why I call it "Surge" (and not "recoil"), please check the post dated 05 July 2018]

So why didn’t I open up my VX100 all this while, like I do with all my other ARs in my continuous effort at altering their states of tune and checking behaviour and performance, you would ask. And the answer to this is that the VX behaves and shoots so mannerly out-of-box that one feels like waiting for it to perhaps go awry at some point, so you might justify the decision to open it up! To most people, it would seldom oblige though, and which really is a vital attribute of a good AR indeed ….Even in my case, but for two things, namely the low ME and the irritating surge on each shot, I may have postponed this tune up! (None of my ARs, including those shooting in the 10 to 12+flb range, have this kind of secondary recoil that is so predictable of the VX. All, by the way, are of similar weights. I must also admit that I was looking definitely also to explore enhancing its ME output.

Having said that, forums have been abuzz about the actual ME of PH guns being way too lower than what is claimed. Just to confirm, my VX, in its factory tune, and post over 2000 shot cycles, was performing between 6.96 to 8.57flb with various weight and fit pellets…all with comparable accuracy at short ranges. The highest figures surprisingly come not from one of Precihole’s own pellets but a US one (The Super Pointed which weighs the same as the PH Sting). This yet again confirmed what I have been saying about the low start pressure attribute of the new lot US pellets, by the way. [Please refer to some photographs attached to this post]

Sub-optimal Energy Efficiency
Low ME output is just one aspect though. Knowing PH springs as the back of my hand (I use and regularly swap several of them), and the bore and stroke of my VX, it was easy to calculate - much disturbingly though- that its energy efficiency in the stock set up was below 30% (29.96 to be precise). With just five-sixth of the spring energy, and with pretty similar PL Force to Force at Release ratios, my tuned SDB27 in .177 dishes out almost the same ME (8.52flb) with a decent 35% efficiency….and an almost recoilless cycle - evident in the better groupings it gives than the VX.

What did the re-tune achieve?

The above note tells you the mind set with which I approached this retune on the VX. The results first: I managed ME increase between 123 and 131% across three different pellets of varying profile and weights, with the highest ME at 10.52flb once again recorded by the US Super Pointed 9.6grn. The recoil behaiour was much evened out, albeit a wee bit snappier, than before. Accuracy as you can see in the photos attached, is at par with that in the previous state of tune.
However, that is just one part of it. The more pertinent aspect is that the efficiency trudged only just over two percentage points to 32.27%. Even for a .177, I would strive for more….and hence the quest will continue with other states of tune....wherein I will play around with other aspects! In any case, I stick to the basic tuning mantra the best I can: If you alter more than one aspect of a set up at a time, you will never be able to know which contributed to the resultant behaviour of your gun. So, do your pre calculations prior to tuning, and just alter one at a time.

A note for the more serious tuners: The stock spring at 267mm FL offered 58mm preload in the stock set up. I replaced this 29.84lb/in with a 278mm FL aftermarket one of comparable SR 30.24lb/in but with coil OD and no. of active coils at 19.3mm and 35 as against the 19mm and 38 active coils of the stock spring. As a result, even though with matching stiffness, the 70mm preload offered by the latter increased the spring energy from 35.08 to 42.16lbf, and preload energy from 6.48 to 9.56lbf. The resultant piston energy shot by 4lbf to 32.60. Overall, the PL Force as a percentage of the Force@Release went way up to 47.62%. Too bad for my taste! I like to keep it in the 40% bracket to achieve optimum peak compression ratio and pressure (And bravo! that's how it was,too, in the factory tune!!).
But WHAT WHEN YOU HAVE THIS MONSTER PISTON to tame!! To ensure a decent pellet driving pressure pulse before piston bounce, the force over the piston must be enough to keep in near or in the vicinity of the TP as long as possible. And you sure will need a lot of preload to keep this heavy piston there!

Not everyone would be interested in these finer aspects of tuning, so lets get back to issues of common interest….and look at some key aspects that stand out on this gun (most likely as well for the SX series, too).

1. Stroke: AT 77mm, it’s just shy of a typical short stroke AR. Why so short, you may ask! My reading is that PH has an evident leaning towards competition target shooting sport (as against promoting solely sporting air rifles).. where accuracy and low range performance are key. ME gets ruled out rather squarely so long as you are in the 6.5-8flb range, which the PH guns dish out any day. But for accuracy, short strokes are very forgiving. In competitions, they wouldn't allow a high ME gun anyway. So, a short stroke seems to work out good.

2. Bore:Stroke ratio: At below 1:3, I believe it’s sub optimal, (but I believe this would be more out of commercial considerations than anything…..so expect PH to come out with a longer stroke gun …in the same power plant!! No alterations other than a different piston as far as I can see. So, you have a winner commercial viability! That's also how it is on most popular Air Arms or Weihrauch models. For the PH guns, it is very much DOABLE from what I see in the comp tube and the action overall!!)

3. A typical energy lean set up: The soft and long spring (with SR in the range of 29.84lb/in with a full set FL of 260mm), the seal fit attribute and the final preload (with full set) working out to two inches….you can’t ask for anything more spot-on. Kudos to the designers at PH for that!

4. Heavy piston: At 284gms (piston, tophat and seal), I find the assembly supremely heavy and out-of-this-world! I am not aware of any pistons in popular adult air rifles in the world being anywhere near this range (not claiming though that I know many of them). Most have been found to work best in a much lower range....the SDB air rifles, to just cite a local example, have got that pretty right there!
[Pls refer to the post dated 6 Jun 2018 for further elaboration on this point...]

5. Limited scope piston: With its ID in the range of 19.4mm, it seriously limits your spring options. Very few aftermarket springs offer an OD to fit into this piston. Please note that it is the final OD at coil bound stage that you should consider here (and not just the ID at free length). Also, the rather longish (15mm) bottom bearing area not only adds to the weight but more importantly, negates scope for adding preload washers over guide.

6. Spring Too soft and LONG: I believe the PH spring is one of the most consistent. Not one of the several that I have played around with have budged beyond 260mm as their full set FL; none weighed anything over or under 115gms; and all except one of these 3mm wire springs had 19mm OD and 38 active coils. Now, if you compare this with a similar spec spring, say a G Smith 27, for example, you will notice that the stiffer latter spring (SR close to 33lb/in) with just 34 active coils perhaps has a higher modulus of rigidity. The final set FL of this 34 coil spring is 15mm more than the 38 coils PH spring. That’s what shorter length wires (read greater pitch) can do to your energy calculations. So, please do insist on getting these right before you attempt any tune up!

7. Limited spring room and preload room: DO not ever think of adding any more than 3mm of slip washers over the guide lest your piston bottom out even before it engages the sear. That is a serious limitation. The alternative lies in utilizing some 10mm of room available inside the piston…but that will further add to your piston weight, so it is a questionable option. Also, it will likely push the stock spring to coil bound state, as well as squeeze the rear guide too close to the tophat on cocking (both undesirable!!).

8. A soft set up: A great attribute on its own. No surprise, the gun feels smooth in its handling and cocking. Here’s a testimonial to this: Coming out of the action the stock spring was 267mm FL, and that’s after 2000+ shots. I had a bet with a friend that it would take a final set at 260, and that he wouldn’t be able to budge it beyond that ever! He just wouldn’t believe….and was game for the bet. After all, 2000+ shots are no joke …They can tame the stiffest springs, he believed! Just over 24hrs of scragging, and the spring was at its sweet 260. Now, I know that if it’s a quality spring, it will not budge beyond say 2mm (yes, two mm) even when kept coil bound for 1000 hours (you read it right, One Thousand hours!). An archaic, but a sureshot, way of verifying spring quality.

9. And finally, the SCR: To be honest, I have a serious hunch that this gun originally was designed to be a longer stroke one, and that the current 77mm models are only an interim offering….say, to get users into the groove first so they can be ready for more later! Aka, a Di engine as the first offering and a common rail direct injection follows some years later! Why I say this is that the TP of the VX is rather classical at 3X11mm. Now, vis-à-vis the current swept vol of 47.42cc, simple calculations will give you a whopping SCR of 609; and you will damn the designers for having committed a blunder. But that is where most of get it wrong. Just add the various grooves, nooks and corners atop the piston head and the cylinder wall, and it drops down to a respectable 450 (Notice those depressions in the cylinder end wall in the attached photo...all of that adds to the lost vol.).

With this TP dimension, I believe 500 should be the ballpark figure. Now, just increase the stroke length to say 85mm to get a swept vol of 52.34cc, and lo and behold, you are right there at the magic SCR of 523!! More reason, why the gun was originally designed for a longer stroke!!

So what holds the gun from delivering outstanding efficiency??
Well, if you notice closely, we have just stated the reasons above! And so, without making it long, I will just state my opinion in points:

1. An exceptionally heavy piston. [While it is able to use less spring energy to create a desired momentum and generate piston energy, the same soft spring even with a respectable preload is unable to tame the bounce securely….and hence you get the heavy surge in every shot]

2. The longish spring. At 120mm coil bound length, I wonder if it bodes well for a powerplant with just 131mm of spring room at lock up.

3. Though pretty much an industry standard TP, its dimensions seems much LESS suited to the current stroke.

4. And finally, the stroke. Take this in at least the 1:3 league, and you could put your money on the gun any day.

What options to try?

If you are into back garden plinking or target shooting, my advice would be to leave it alone. If you can manage good groups with this high surge gun, you can shoot any in the world. So, just master your hold technique!

But if you are into other forms of the sport, then, here goes…
1. The easiest option is what I did: install a spring that offers greater natural preload, and yet has smaller coil bound length. If its WD and CD are comparable to the PH one, it’s bound to be stiffer.

2. For the more seasoned DIY buffs, attempt two things: remove coils (say three, to begin with) from you stock spring to take its stiffness to some 5.6N/mm. But here you will need some 20mm of extra preload to increase the force at release. Manage this by removing the steel tophat, installing a light weight polymer tophat and adding slip washers (preferably derlin/polymer ones, to minimize weight). This will additionally work by lessening the sectional density of the piston…and thereby enhancing piston velocity during the compression stroke, and a lighter piston would be easier to be tamed by the stiffer spring during bounce. You may just have to watch out for the second landing velocity though (It can be a bit harsh in this case).

3. Reduce piston weight. Looks intimidating…but can be pretty easy going if you know how. Will do a separate post on this later.

4. Increase the stroke. Ditto as point #3….but very much doable. Do you have access to a mini lathe?? Great stuff for my next project. 85mm looks achievable….and should be a good case study for all of us! Let me see how soon I get to go at it!

Hope this note helps us make inputs, contest, qualify, refine or detail out any of the points above, and eventually enrich our understanding of this popular Indian air rifle.
Cheers!
Prashant

[Bullo oka]

Wow prashant, thats the whole lot of info.
It would be very kind of you lads if you make some post like this on Gas piston/Nitro piston.

Sent from my ONEPLUS A5000 using Tapatalk

[Rudransh]

Dear Prashant hi
Nice and well worked up job u did on Prechihole. Such kinda job and tuneup was very common in the ages of our beloved IhP ( May god grant it eternal peace).

But after introduction of Prechihole and its initial trigger escaping behaviour once these initial problems settled down Prechihole became most desired airgun in our small cult.

It's was just fabulous out of the box that people who had encountered other Indian airrifles at that time fell in love with it instantly, And honestly it's worth that kinda attention.
Gradually with passages of time other companies also came in market with their improved version of airrifles it was needed to face the brutal challenge put by Prechihole quality ( thanks to Doc) so AARMR,SDB,AK also came with better products to meet the expectations height introduced by Prechihole,.

After few genuine competition coming into frame people like us started comparing Prechihole to these other Indian companies.In every aspect Prechihole leads to other companies but always lacked in the velocity figure sometimes we thought we got a bad piece maybe but sx never reached 800 fps with real world pellet same thing happened to vx I don't think why they need to tell us 800 fps when it gives way lower than the this.

A general question bugs out my mind that any airgun if tuned down will perform with same smoothness as a vx. But I am glad that now we can see that there is a chance to improve it for more ME with same smoothness.
Keep us updated that this tuneup holds after few hundred shots.

Best of luck.
Regards
Veerbhadra

[Bullo oka]

I think they should produce the non club rifle with 20joules instead of 16.(16joules of precihole isnt 16, its lower than that. But if they make 20joules, atleast it will be 16)

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[Basu]

Dear Prashant,
You have taken the matter to such a depth , I find very difficult to reach.
Having said so , one thing PH springer or Nitro bothered me was it's snappy recoil.
IFGian Pratik Mahale tried some experiment by removing top hat.
This might give some relief of snappy recoil but he noticed that that also resulted in reduction of velocity.
I am of the opinion that if the TP is changed to 11x3.2 mm then the snappiness of recoil will be reduced.
I had tested the vibes of PH Club on vibro meter that returned 5.7 on Richter scale as against 4.6 of SDB 45 & 3.8 of National 25.
Hope reference to the above testing is already there on the forum.
I invite Pratik Mahale to throw some light on the subject.

Basu

[prashantjha19]

Dear Prashant hi
Nice and well worked up job u did on Prechihole. Such kinda job and tuneup was very common in the ages of our beloved IhP ( May god grant it eternal peace).

But after introduction of Prechihole and its initial trigger escaping behaviour once these initial problems settled down Prechihole became most desired airgun in our small cult.

It's was just fabulous out of the box that people who had encountered other Indian airrifles at that time fell in love with it instantly, And honestly it's worth that kinda attention.
Gradually with passages of time other companies also came in market with their improved version of airrifles it was needed to face the brutal challenge put by Prechihole quality ( thanks to Doc) so AARMR,SDB,AK also came with better products to meet the expectations height introduced by Prechihole,.

After few genuine competition coming into frame people like us started comparing Prechihole to these other Indian companies.In every aspect Prechihole leads to other companies but always lacked in the velocity figure sometimes we thought we got a bad piece maybe but sx never reached 800 fps with real world pellet same thing happened to vx I don't think why they need to tell us 800 fps when it gives way lower than the this.

A general question bugs out my mind that any airgun if tuned down will perform with same smoothness as a vx. But I am glad that now we can see that there is a chance to improve it for more ME with same smoothness.
Keep us updated that this tuneup holds after few hundred shots.

Best of luck.
Regards
Veerbhadra

Indeed, competition is a great thing to have caught up. And in the natural course of things, it should be a popular SDB model or so in the near future that we would be looking at examining closely.
In fact I have worked extensively on and have details on some of their older models that I own and use. But people are mostly now hung on to the newer ones Artemis, Sport, and all. So, am planning to take on one of these soon.
Best regards,
Prashant

Added in 12 minutes 9 seconds:

Dear Prashant,
You have taken the matter to such a depth , I find very difficult to reach.
Having said so , one thing PH springer or Nitro bothered me was it's snappy recoil.
IFGian Pratik Mahale tried some experiment by removing top hat.
This might give some relief of snappy recoil but he noticed that that also resulted in reduction of velocity.
I am of the opinion that if the TP is changed to 11x3.2 mm then the snappiness of recoil will be reduced.
I had tested the vibes of PH Club on vibro meter that returned 5.7 on Richter scale as against 4.6 of SDB 45 & 3.8 of National 25.
Hope reference to the above testing is already there on the forum.
I invite Pratik Mahale to throw some light on the subject.

Basu

With the same low rate spring, removing the tophat will for sure bring down the velocity (and hence momentum) of the piston, Basu da. No wonder the loss in ME.
In fact for the given stroke, the 3.2mm dia suggestion for the TP sounds spot on. For more precise calculations, however, one would need oscilloscopes and pulse measuring rigs. Late Prof. Emeritus M Wright's three formulae for this relate to matching the pressure pulses, matching pellet and pistn velocities, and one more that I am forgetting now. However, even the results from these three calculations vary by small margins. So, TP remains an enigmatic area as ever.
Thank you for confirming the recoil behaviour of the PH guns on the vibro meter. Much needed validation!
Also, with a higher rate spring, the top hat can be done away with....and has been found to give a good shot cycle behaviour.
Best regards,
Prashant

Added in 9 minutes 53 seconds:

Wow prashant, thats the whole lot of info.
It would be very kind of you lads if you make some post like this on Gas piston/Nitro piston.

Sent from my ONEPLUS A5000 using Tapatalk

I am told the powerplant specs are the same for bothcoil spring and gas rams in PH guns. Also, there's very little you can alter in gas rams apart from the fill pressure....which is usually rather too hot, specialised and sophisticated a jig to handle for shed tuners, you see. Also, once set, they are best left alone. Some bit of playing with preload can be done and will be good to know if PH does leave any scope for that. Just be mindful of leaving at least a 2 to 3 mm leeway so the full stroke never ramps the pressure up to max.
Best regards,
Prashant

[vsraja]

1)Precihole has longer transfer port.It is a negative in two aspects,as it reduces the amount of compression received by the pellet and affects the swept volume and hence hard to achieve the power claimed.
2)Then it has to design a different piston seal which covers the piston fully.It also helps to smoothen the firing cycle when it hits the front of compression tube.
3)Also precihole can adapt to ball type locking system like in diana air rifles.This type of closing system needs less pressure to close the barrel even without snapping sound.

[prashantjha19]

Interesting and valid suggestions those all, dear VS! Yes you could take the TP length down to say 6 or 7 mm with this dia...but wont that make the cylinder wall rather flimsy?
And a Weihrauch type full dome parachute seal could be ideal ....but pls also look at the costs involved. In any case, the current seals haven't failed me ever. And for a mere Rs 34 a piece, what else would you ask for? Yes, there is a depression 1mm deep X 10mm dia on top adding to the lost volume, but that is also helping absorb a lot of the landing jerk, you see. In any case, I believe the quality of the silicone used, its thermal coefficient and the lip flaring are the key traits of a great seal. As on date, the one in use is going good...but there's no end to improving!
The ball barrel lock mechanism would indeed be something to look forward to.
Best regards,
Prashant

[vsraja]

The main concern behind my idea of using a full covered parachute seal is mainly to stop the slamming of that steel front end of the piston on the receiver.The clearance area between the front end of the piston and the receiver is very less and will definitely hit the receiver every time. This metal to metal slam should be avoided.

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[Sombit]

Prashant jha, it was a very informative post. But there are some people like me who doesn't have any experience with tuning. Can you please upload a video to show us what you did? It would help a lot.

[baggafx]

Veerbhadra

Finally i questioned Precihole about VX-SX 100 12 FT-LB 800 fps claim & they Replied see this topic...

viewtopic.php?f=2&t=26071

[Maroon_d3vil]

Wonderful Prashant Ji! Bookmarked for future.
Regards,
Maroon_d3vil

[prashantjha19]

The main concern behind my idea of using a full covered parachute seal is mainly to stop the slamming of that steel front end of the piston on the receiver.The clearance area between the front end of the piston and the receiver is very less and will definitely hit the receiver every time. This metal to metal slam should be avoided.

Sent from my SM-G610F using Tapatalk

Dear Vsraja, Pls rest assured...the piston will NEVER slams on the cylinder wall ...neither in a dry fire, and certainly not when there is a pellet in the breech. Also, in the Precihole action, the air cushioning provided by the seal's central lip area sitting over the piston button head is pretty secure and hefty, ...and will certainly not allow any slamming. In fact, as you would have noticed in my original post, this gap seems to have been deliberately worked in by the designers to expand the lost volume (the advantage being that the TP does not need to be tampered with...).
A full cover parachute seal (which usually have a flat face with just a flared lip) might not, in my opinion, allow this kind of a scope for lost volume.
Regards,
Prashant

Added in 57 minutes 43 seconds:
4. Heavy piston: At 284gms (piston, tophat and seal), I find the assembly supremely heavy and out-of-this-world! I am not aware of any pistons in popular adult air rifles in the world being anywhere near this range (not claiming though that I know many of them). Most have been found to work best in a much lower range....the SDB air rifles, to just cite a local example, have got that pretty right there!....

Hi there!
Somebody, having read the original post, connected with me with some queries, ....in particular, the one quoted above...the Piston weight issue! His dilemmas made me realize that ...very often, in order to keep such detailed posts from being longish, one has to avoid or prune so much information. Yet, sometimes, some necessary bits get left out. It seems to be classic case of this ..on the piston weight issue........And hence this add-on post please!
The good thing though is that we have many who do brood over these technical aspects. And so, we have a duty to clarify and share whatever little we know...

Well, lets first have the GOOD NEWS: We learn from authentic sources that a LONGER STROKE gun is in the offing from Precihole Sports, most likely in the 20Joules range ....and that, too, not so much in distant future!! So lets eagerly look forward to it!!

Now, the piston weight juggernaut...
Well, ideally, while commenting on the VX piston weight in my original post, it would certainly have been more accurate (and prudent) for me to state that it is "....supremely heavy....given the stroke!" I however seem to have ignored the latter bit. Please remember that this weight must always be read in consonance with both the BORE and STROKE of the action. This is because, while the BORE dictates the sectional density at play during the compression stroke, it is the stroke that significantly determines not just the lock time, and the potential cylinder pulse (the fraction of the stroke when the piston and pellet are travelling in the same direction during which the pellet gains most of its energy), and the peak pressure and temp developed during the compression stroke.

Let me take an example from a recent gun, the sub-12fpe Walther LGV. A terrific shooter which, since its launch a couple of years back, has been giving the most favourite ones in the HFT circles, namely the heavier TX, the HW97 and its own sibling the LGU, a real run for their money....despite it being a lighter gun (read potential for more recoil) and a break barrel (against all those underlevers)!!
Now, this gun's piston dynamics in the action would (its weight is in the 250 gms range) work out out pretty similar to how the VX's piston behaves. And yet, you would hardly ever hear a whine about the noticeable surge problem we mentioned about the VX). Why??

Well, firstly, with a much lighter tophat, the sectional density of its piston assembly in the 25mm bore of LGV works out pretty similar to that of the VX's. But a longish stroke at 88mm is what may be working to neutralize any negative effect of a high sectional density in its case. Reason why we opined that a longer stroke might give a much smoother shot cycle in the VX.

Another thing worth noticing is that while a heavier piston would favour heavy pellets (shorten the stroke further and /or increase the static compression ratio by tampering the TP, and you have a PCP like behaviour!!)....it will also be much less prone to relying on dieseling to produce a given muzzle energy. As you can see, there are advantages of a heavy piston.....albeit the downside is the potentially high secondary recoil (surge) and, in some cases, even piston slam on second landing.

I hope this has been able to answer some of what may have been nagging many of us.
Best regards,
Prashant

[baggafx]

percihole accepted that there SX 100 did not produce 12 ftlb power in .177...sx 100 produce 800fps with 7.3 grain pellet 10.38fpe.... again when i ask is SX-VX 100 is same then they reply VX 100 is almost same but produce 760 fps with 7.3 grain pellet mean 9.4 ft-lb power...see email pics

[Rover solomon]

Very nice attempt