Measuring Rate of Twist in Your Rifle

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MEASURING RATE OF TWIST IN YOUR RIFLE

by Paul Vallandigham

Learn to measure the Rate of Twist yourself. Then there will be NO DOUBT about it.

Put a wire bore brush on a cleaning rod, then put a lubed Patch on the muzzle, and run the patch down the barrel with the bore brush. Go to the bottom, pulling back every once in awhile to make sure you can pull the brush and patch out again. ( Always twist the brush clockwise to help the bristles adjust to move backwards, when you pull the rod back out.) The lubed patch helps the brush run down and back out the bore. The Bristles of the bore brush will hold the patch in place, and stick through it to get down into the grooves.

Now. Mark the rod at the muzzle with tape, or some other way, at the muzzle. Use your front sight as a reference. The easiest way is to make two "ends" to the tape, that come together and stick up off the rod, in line with that front sight. But, a mark on the tape works as well.

Twist rates are measured in so many inches per Revolution( i.e. how much distance in the bore does the ball move to rotate 360 degrees). With Round Ball barrels, the ROT can be slow enough that you won't get a full rotation inside the barrel. But, a full rotation is NOT needed to determine the ROT. You can determine the entire ROT length by rotating the brush just 1/4 turn.

With the tape on the rod, marked to index your front sight, slowly pull the rod out, feeling the rod so that you know the bristles are staying in the grooves. When the rod has turned 1/4 turn or rotation, stop. Measure the distance from the tape to the Muzzle. Assuming the tape was wrapped around the rod right at the muzzle, the distance from the tape to the muzzle at the 1/4 turn position will be 1/4 of the length of the ROT. So, multiply the distance measured from the muzzle to the tape, and multiply that number by 4 to get the full ROT.

Do this measurement several times, as its easy, at first, to grab the rod too hard, and get a wrong reading.

I walked my brother through this process over the phone a couple of years ago, with a new gun, and he came up with ROTs that made no sense- odd numbers not even close to the standard ROTs expected. I had him repeat the exercise several times, until he got a feeling for the rod and brush, and stopped holding the rod so hard. Then he got a proper measurement, and repeated the same measurement 3 times in a row.

Boy, was he happy to learn how to measure his own ROTs. He told me that he didn't really understand WHY I was telling him to Not hold the rod so hard, when he pulled the bore brush back out the barrel UNTIL he learned to do it correctly! I laughed. It was the same experience I had the first time I learned to do this. And, the friend who showed me how to do my own measuring was on me to " try it again", just as I had done with my Brother.

Why not just ONE ROT per caliber? Well, some want to shoot conicals, others just Round Balls, and others want to shoot both! The slower the ROT, the more forgiving the barrel is to changes in powder charges, however. That is the reason you see so many RB barrels of different ROTs.

A glance down the barrel with a bore light will give you an idea on how deep the grooves( rifling) are in a barrel. If the grooves are shallow, its a fast twist barrel intended to be shooting conicals. It will shoot a Patched Round Ball, but at reduced powder charges for accuracy. The shallow grooves work with a bullet that has a lot of surface area pushing against the grooves. The surface area of a Patched Round Ball, that actually pushes against the rifling, is too small to get a good grip, particular at higher velocities.

Conical bullets are often sized only .001-.002" smaller than the bore diameter of the guns. Sometimes they are paper patched, but generally, a conical bullet sized that close is expected to expand on firing to fill the shallow grooves to provide spin to the bullet.

A deep grooved barrel is a RB barrel, designed to shoot a Patched Round Ball. The deep grooves provide a place for the cloth patch to go when the fabric is compressed by the lands( bore) of the rifle. The fabric pushing into the deep grooves then provides a better seal to keep gases behind the ball.

By " Deep", we are talking about groove depth that is .008"- .012" deep from the land next to it. Shallow-grooved barrels will have groove depths from .002-.005 or .006". Compare your MLer barrel's rifling to any modern pistol, or rifle barrel to see the difference, or similarity, in groove depth.

What should you expect for Rates of Twist of rifling in Muzzle loading rifles? The faster ROTs, used in shallow grooved barrels designed to shoot conical bullets will be in the area of 1:28 to 1:34. RB barrels, with deep grooves, run from 1:48 up to 1:70, with some large bore barrels being rifled with ROTs as slow a 1:100 !

{NOTE: Pistol barrels will often have faster ROTs to put more spin to the bullet or PRB in the shorter length of those barrels.}

The 1:48 ROT is considered a " compromise ROT" for guns of .45 to 58 caliber, as short, lighter weight conicals can be shot accurate with that fast a rate of twist, while that rate of twist also allows PRBs to be shot accurately from these calibers. Typically, the 1:48 ROT barrels will be made with deep grooves, to help in shooting PRBs. For best accuracy using conicals, some kind of filler, or Over Powder wad is recommended to seal the gas behind the bullet, and protect the base of the bullet when it leaves the muzzle of the gun.

Paul Vallandigham, 10/09
 
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