10.7 FPE Air Rifle SN-45

[Basu]

http://indiansforguns.com/viewtopic.php?f=2&t=22389


This is in continuation to my earlier post wherein I had given the out of the box observations on my new breed gun ,SN-45.

This gun is SDB45 Classic except its barrel and back sight are from IHP and so requires serious tuning.

The tuning process:

Disassembled the gun first.
The main spring is of 40 coils and blinded by rage due to its hard cocking, punished the spring by kept it in cocked position first for 12 hours then again for 6 hours with a gap of 6 hours.
Removed two coils and kept the free length at 28.7 cms with 38 coils. Both ends were grinded and finally smoothened on sand papers. The gap between the coils measured to be 5 mms.
The piston was completely dark with all dust and rust which was polished by 320 grits sand paper both inside and outside to give a gentle look.
It has oil-tempered sear rod which helped me to give mirror finish with 92-93 degree taper.(Caution : please do not try this unless you are quite at home with this job.)
The depth of the sear is 3 mm, which I wanted to reduce to 2.5mm yet it could not be done due to hardness of material.
The interface of the trigger is given super mirror finish with same 320 grits.
A Coke bottle part used as sleeve inside the piston wall to stop buzz of the main spring.
A thrust washer is put through sear rod to offer a nice surface to spring to get easy torque.
Polished the inside of receiver with handmade honing tool with 400 grits sand paper.
Fitted last TTI commercial seal on piston, softening by putting it in hot water.
Cleaned every part in kerosene oil.
I was desperately in need for moly but did not have a pinch of that at all.
One IFGian Kaushik had given me local moly, certainly no where close to 60% Mos2, but good enough for ARs, were used sparingly on spring and piston rear bearing.
A specially made spring guide is used with OD of 13.5 mm to match the main spring ID.
A thin layer of this moly is also given on the guide.
The cocking shoe is filed to remove uneven surface.
TTI seal received a little nick on lip while putting in receiver. The seal then resized and put back again.
It was little tight fit inside the receiver.
It is noticed that with synthetic seal the piston stroke increased to 8 cms and so swept volume went to 49 CC.
The breech seal is replaced with Welding hose and kept 0.5 mm protruded.
I was longing for Precihole used seal but that was not available at that moment.
The advantage with used seal is that, it starts giving maximum FPS from very first shot unless too misfit.
The entire assembling was done and now it was ready for testing.

Velocity:
Unitedshot 6.8 grain---------------- Max 840 FPS with pellet pusher , no increase.
Gsmith amateur 8.2 grain-----------Max 739 FPS. With pellet pusher 772 FPS.
Mastershot Flat Head 7.7 grains—Max 740 FPS with pellet pusher 775 FPS

840 FPS was really palpable at 40 yds, taking out dust from wall every time.
All above datas of Chonoconnect.

Noise:
74 dcbls at 5 mts from muzzle, not noisy.

Vibration:
4.2-4.3 Richters with all pellets. Quite good for accuracy

Accuracy:
Not very good, trigger is not that smooth as of now. It is not engaging at the same place every time.

Cocking Effort :

Less than earlier pull, manageable.

Over all feel:

Much smoother yet the need for quality moly is felt.Easy to shoot. The seal yet to get set.

Conclusions:

The gun is still breaking-in. I suspect that it has capacity to deliver a little better FPS due to long stroke.
The piston weight is in the range of 260-270 gms, due to which it gives low recoil.
The improvement in accuracy is on the card once it passes 1500 shots.
I have to wait till then…………

Basu

I request SathyaSniper to post some photos on my behalf and oblige.

[sathya.sniper]

Posting this on behalf of Basu sir,
These are some of the picture during the tuning of the rifle
Honing tool for the chamber.

Piston as removed from the chamber

Piston after polishing with grit paper

Piston seal (Apex seal)

Spring Guide with Spacers...... No of spacers as required.

Breech seal made of rubber instead of Leather seal.


Velocity readings




Accuracy
This Rifle is still breaking in so it needs some time to settle down. And some of the last shots at the bottom started to group better will get better as it settles down.


Regards.

[pratik_mahale]

Good Job Sir
keep it up

[brihacharan]

Great Job Basu
You claim that you gained "8 cms" in piston stroke?????
8 cms = 3.14" - that's quite a lot & that too because of the TTI's seal....incredible
Does this mean that the original seal was 3.14 longer than TTI's seal???
Interested to know how you estimated the difference in the stroke length......
Briha

[bennedose]

Good job Basuda - I am glad you had success in this endeavour which we had discussed earlier.

Incidenatlly what do you think of the SDB 65 rifle? Any informed view?

[Basu]

Dear Briha,
With leather seal the AR had stroke length of 7.6 cm.
With TTI seal it further increased by 0.4 cms ,thus making the total stroke length 8.00 cms.

Dear benne,
Your prescription to punish the spring for longer period ,gave me confidence and so the 1st grouping might have gone wayward but subsequent to that it has improved rapidly.
Hit pencil cell three times at same place. whatsapp pic.with sathya and Briha.
SDB 65 looked front heavy with extra attachment as muzzle break. 'C' lever is nt that nice.The rear sight is not Diana type.Specifications should be as good as SDB 45.

Dear Pratik,
Had I used stock spring it its formar state with increased weight of piston , hope the gun could have crossed 11+fpe mark.But I was just not interested.

Basu

[pratik_mahale]

Smoothness is the key point for accuracy
Don’t compromise accuracy for FPE
You can get preciholes moly grease from Amico it’s a good investment to do

[brihacharan]

Dear Briha,
With leather seal the AR had stroke length of 7.6 cm.
With TTI seal it further increased by 0.4 cms ,thus making the total stroke length 8.00 cms.

Dear Pratik,
Had I used stock spring it its formar state with increased weight of piston , hope the gun could have crossed 11+fpe mark.But I was just not interested.
Basu

Hi Basu,
> Thanks for the clarification - now it makes sense
> Like Pratik has said - An accurate AR is any day worth more than the one that produces more fpe with less accuracy!!!!
Cheers
Briha

[Basu]

Just an update.....
SN45 performed better but just not as consistent as it shuld be.
Opened the gun again , cut the main spring ,kept it at 28 cms free length or 36.5 coils.
Increased the taper of sear by one degree more.
Used Prechole moly and used piston seal ,very graciously given to me by Kaushik Mitra ,IFGian(.22cal).My thanks to him.
AR turned dead accurate with easier cocking .

Velocity :

Gsmithamateur- 715-741 fps
Mastershot flat - 730-752 fps
Unitedshot pointed -764-796 fps

U may notice the velocity increased by a little margin.
Precihole moly is good but could not remove all scraping sounds.
Any body , who used TTI moly , can make the diffence.

There is a little change in sound and went louder.

The POI found diffent for MSFH and GSFH.
Hope this is normal.

I have reached my objective.



Basu

Note : All above datas are of chronoconnect Samsung tab 2 with headset.

[bennedose]

Basu reading through Cardew's book - there is an interesting bit of information which goes against "common knowledge" on this forum.

The Cardews point out that a very small amount of dieseling is an integral feature of ALL springers and
1. They have photographed and documented a flash caused by this in normal shooting
2. They could only prevent this by excluding all oxygen (by housing the rifle in an atmosphere of nitrogen only) - and this lead to a significant loss of power.

My contention is as follows, based on Cardews' comprehensive work it is not necessary to bend over backwards and use Moly grease alone. Other lubricating oil eventually finds its way to the compression chamber via other routes. As long as you are not deliberately charging the pellet with a drop of oil - a small amount of oil and some dieseleing is part of the springer's performance.

[Basu]

Yes benne ,very true .
When I opened the gun ,I found the lubes went into the chember by taking the lower route of piston slot.
This may burning when compression is taking place.
Another important fact I observed that even after cutting the spring further ,the velocity increased by a little margin except for Unitedshot 6.8 grns pointed.
MSFH gave 829 KMPH and the same was not there with full 40 coil spring.
Hah ha hah !!


Basu

[bennedose]

Another important fact I observed that even after cutting the spring further ,the velocity increased by a little margin except for Unitedshot 6.8 grns pointed.
MSFH gave 829 KMPH and the same was not there with full 40 coil spring.
Hah ha hah !!

Basu

Which goes to show that a muscular guy pumping harder would not able to get someone any more pregnant than a thin guy putting in modest cocking effort.

[brihacharan]

Which goes to show that a muscular guy pumping harder would not able to get someone any more pregnant than a thin guy putting in modest cocking effort.

Wake up "Velocity Crazies" .... The truth is knocking on your doors
Briha

[Rrahulkumar]

Another important fact I observed that even after cutting the spring further ,the velocity increased by a little margin except for Unitedshot 6.8 grns pointed.
MSFH gave 829 KMPH and the same was not there with full 40 coil spring.
Hah ha hah !!

Basu

Which goes to show that a muscular guy pumping harder would not able to get someone any more pregnant than a thin guy putting in modest cocking effort.