brno microgrooving

[mahboob]

Brno 22 lr :what models are microgrooved and how to check a particular rifle for microgrooving? pls help.

[brihacharan]

Brno 22 lr :what models are microgrooved and how to check a particular rifle for microgrooving? pls help.

Hi Mahboob,
> I came across this wonderful write-up about rifling / micro grooving & want to share it with those who want to know more.
THE RIFLE BARREL - By Chuck Hawks
A rifle barrel is usually a long (16 to 30 inches), tapering, tube made of steel with fairly thick walls. It starts as a steel bar, which is then drilled and reamed to the "bore diameter" of the cartridge for which it is to be chambered. For example, the bore diameter is .300 inch for a typical .30 caliber cartridge like the .30-30, .308 Winchester, .30-06 or .300 Magnum. The rifling grooves are then cut into the bore. The distance across the grooves is called the "groove diameter." This varies depending on the caliber, but measures .308 inch for typical .30 caliber cartridges.

The outside of the barrel is tapered by turning it down to whatever contour is desired. This lightens the barrel by removing excess metal. Tapered barrels are greater in diameter at the chamber than at the muzzle, because when a cartridge is fired, the pressure is highest at the chamber end. Some barrels, called "bull barrels" are not tapered at all. These very heavy barrels, designed for extreme accuracy, are usually seen on target rifles.

MATERIALS AND BARREL LIFE
Rifle barrels are usually made from steel alloys called ordinance steel, nickel steel, chrome-molybdenum steel, or stainless steel, depending upon the requirements of the cartridge for which they are chambered. The higher the pressure and velocity of a cartridge (pressure and velocity usually go up together), the faster it will wear out a barrel.
To give a satisfactory service life, barrels for high velocity cartridges must me made from tougher and harder steel than barrels for lower pressure cartridges. A .270 Weatherby Magnum requires a tougher barrel than a .270 Winchester, which requires a tougher barrel than a .30-30, which requires a tougher barrel than a .22 LR. So manufacturers generally use more erosion resistant steel in barrels for high intensity cartridges, and barrels for magnum cartridges are often made of very hard stainless steel. Harder steels are more difficult to work and raise manufacturing costs (and thus retail prices) which, of course, is not desirable. The consumer has to rely on the manufacturer's judgment in this area. Fortunately, the major name brands like Browning, Marlin, Remington, Ruger, Savage, and Winchester all seem to know what they are doing in regard to choosing appropriate barrel material for their rifles and barrel life is longer today than ever before.

BARREL BREAK-IN
The idea that there is some special "break-in" or cleaning routine required for new rifle barrels has gotten a lot of attention recently. My advice is to follow the suggestions of the barrel manufacturer. If the maker of your rifle barrel prescribes a specific break-in routine, it is probably best to follow their instructions.
Otherwise, my approach has always been to sight-in a new rifle normally, without any special break-in. I try to avoid overheating a rifle barrel regardless of its age. At the range, I generally shoot three shot groups with centerfire rifles and let the barrel cool between groups. With very high velocity cartridges, such as the .257 Weatherby Magnum, I let the barrel cool down after every shot. Heat increases erosion and wear in any rifle barrel. I generally limit range sessions to one or two boxes of ammunition (20-40 rounds) and I clean the rifle normally after I return home. I have never found any special break-in routine to be necessary.

RIFLING
The purpose of rifling is to stabilize the bullet to increase its accuracy. This is called spin stabilizing, and works because of gyroscopic forces acting on the spinning bullet during flight. American rifle barrels traditionally spin bullets to the right, but British barrels spin them to the left.
There are various ways to rifle a barrel. The old way was to cut the rifling one groove at a time on a rifling machine. A more modern method is to pull a gang of broaches through the barrel, which cuts the all the grooves into the bore simultaneously. Another is to insert a very hard mandrel, which bears the reverse of the intended rifling pattern, into an oversize bore; then the outside of the barrel is "hammer forged" (or beaten) to impress the rifling into the bore. A fourth method is to pull a very hot rifling "button" through the bore, turning it as it progresses, which irons (melts) the rifling into the barrel. All of these methods are entirely satisfactory if done properly.
RIFLES HAVE BEEN RIFLED WITH FROM TWO TO EIGHT FAIRLY DEEP GROOVES. ANOTHER APPROACH, CALLED "MICRO-GROOVE" RIFLING BY THE MARLIN COMPANY, USES MANY SHALLOW GROOVES. BARRELS FOR STANDARD CENTERFIRE CARTRIDGES OF .30 CALIBER OR SMALLER ARE USUALLY RIFLED WITH 4 GROOVES. BARRELS FOR MOST CARTRIDGES ABOVE .30 CALIBER ARE RIFLED WITH 6 GROOVES, AS ARE .22 RIMFIRE BARRELS.

RIFLING TWIST RATES
The rate of twist, expressed as one turn in so many inches (i.e. 1 in 10"), is designed to stabilize the range of bullets normally used in a particular caliber. It takes less twist to stabilize a given bullet at high velocity than at low velocity. At the same velocity in the same caliber, longer (pointed) bullets require faster twist rates than shorter (round nose) bullets of the same weight and heavier bullets require faster twist rates than lighter bullets of the same shape. It is undesirable to spin a bullet a great deal faster than necessary, as this can degrade accuracy. A fast twist increases pressure and also the strain on the bullet jacket.

Fortunately, the rate of twist chosen by the rifle maker is usually appropriate for the intended cartridge. Anyone ordering a new barrel for a rifle will generally do well to specify the standard twist as supplied by the major rifle manufacturers for that caliber.

Once in a great while, though, a manufacturer makes a mistake. One such case involved the .244 Remington. When first introduced, barrels for this caliber were made with a 1-in-12 twist, because Remington anticipated that their new cartridge would be used primarily for varmint shooting. The 1 in 12 inch twist is ideal for best accuracy with varmint weight bullets (70-85 grains) in a high velocity .24 (6mm) caliber rifle. The heaviest spitzer bullet that a .244 with a 1 in 12 inch twist barrel could stabilize was 90 grains. The customers, however, also wanted to use their new .24 caliber rifles for hunting medium size big game, with 100 grain bullets. Needless to say, customers ignored the new .244 Rem. Remington soon saw the error of their ways, and changed the rifling of their .244 barrels to 1 turn in 9 inches, but the damage was done. Sales remained so slow that eventually Remington had to discontinue the .244. The following year they reintroduced the exact same cartridge as the 6mm Rem. and produced all 6mm rifle barrels with 1 in 9 inch twist barrels, which can stabilize all .24/6mm bullets.

The usual twist rates for some of the more popular rifle calibers are given below.
.22 Short = 1 in 24" / .22 Long Rifle = 1 in 16" / .223 Remington = 1 in 12" / .22-250 Remington = 1 in 14"
.243 Win= 1 in 10" / 6mm Remington = 1 in 9" / .25-06 Remington = 1 in 10" / .257 Wby. Mag. = 1 in 10"
66.5x55 Swedish Mauser = 1 in 7.5" / .260 Reming= 1 in 9" / .270 Win= 1 in 10" / .270 WSM = 1 in 10"
7mm-08 Remington = 1 in 9.25" / 7mm Rem. SAUM = 1 in 9.25" / 7mm Rem. Mag. = 1 in 9.25"
.30 Carbine = 1 in 16" / .30-30 Win = 1 in 12" / .308 Winchester = 1 in 12" / .30-06 Springfield = 1 in 10"
.300 WSM = 1 in 10" / .300 Win. Mag. = 1 in 10" / .300 Wby. Mag. = 1 in 10" / .303 British = 1 in 10"
.32 Win Spec. = 1 in 16" / .338-57 O'Connor = 1 in 10" / .338 Win. Mag. = 1 in 10" / .35 Rem = 1 in 16"
.350 Rem. Mag. = 1 in 16" / .375 H&H Mag. = 1 in 12" / .416 Rem. Mag. = 1 in 14" / .444 Marlin = 1 in 38"
.45-70 Govt. (Marlin and Ruger rifles) = 1 in 20" / .450 Marlin = 1 in 20" / .458 Win. Mag. = 1 in 14"
Not all rifle barrels of the same caliber have the same twist rate. A fellow ordering a custom rifle may have his own ideas about twist, as may the builder. For example, some .270 Winchester barrels are rifled with a 1 in 12 inches twist, some .30-06 barrels are also rifled 1 turn in 12 inches, and some .300 Magnum barrels are rifled 1 turn in 14 inches. Usually these variations make no great difference. These slower twists may give slightly lower pressure, as well as very slightly better accuracy with the lighter bullets in each caliber. They will still stabilize the heavy bullets over practical hunting ranges. However, they might not be such a good choice, or quite as accurate, for shooting heavy bullets at extreme range like 600-1000 yards.
Cheers
Brihacharan

[marksman]

we have have a .22 LR Brno Mod. 2 in our family which was manufactured in 1972 and that also happens to be the year it was bought in. Sometime in late eighties while peeking through the barrel I realized that it was Micro grooved.
I'd like to know if any other IFGian possesses a Micro grooved Brno rifle. I have experienced that the accuracy in rifles seem to be much better with this kind of grooves. Getting one holer group with a bench rest and RWS Match Ammo (R-50)at 35 yds is quite common, with a quality scope. Even the velocity improves marginally which could be due to less friction to the projectile I guess. We have an excess to a Chronograph and intend experimenting and finding out soon.
Marksman