Need help advising friend of the necessity of cylinder and bore measurements.

[azrednek]

Can anybody lead me to an internet article or blog on the need for and the how to's slugging a revolver's bore and cylinder. How it relates to proper boolit sizing to shoot lead accurately. Hopefully not but possibly the need for having a cylinder reamed out slightly. I'm a slow two fingered, hunt and peck typist. So to make life easier for me I'd really appreciate if somebody could point me in the direction of a good article I can forward on to my long distance friend.

[geargnasher]

No better place than right here. I'll take a stab at it, this is how I do it.

Assuming your friend has a boolit mould for the gun already, have him cast some boolits from pure lead. This can be either roof jack lead, shower pan scrap, iron sewer pipe joint lead, some factory roundballs, whatever.

There are three measurements to consider: Groove diameter, thread choke, and cylinder throats (taken as an aggregate).

First clean the bore and cylinder well, and remove any traces of copper jacked fouling with chemicals or lead fouling with a bronze brush wrapped in 4-ought steel wool. Rinse and dry, then lightly oil the bore and throats.

For the Groove dimension: Lightly oil one of the "slugs" and pound it into the muzzle with a plastic or rawhide mallet. It will deform and probably shave, no worry. Once flush, use a 5/16" or larger brass rod to hammer the slug down the barrel, stopping about an inch shy of the place where the barrel enters the frame. Take several short sections of brass rod and, one at a time, stacking one on top of the other, drive the slug back out of the muzzle from the opening in the frame.

Next, the "thread choke". Same as above, but drive it all the way through the barrel and out the forcing cone.

After that, see if the as-cast slugs will press through the cylinder from the front with thumb pressure. If so, take six of them and "fatten" them a bit by placing on an anvil and hitting them with a hammer. If they are a tight fit, press/tap them through the throats, one per throat, marking each one and the throat through which it was passed.

The interpretation I make: If the groove diameter slug is more than .0005" (half-thousandth) larger than the one driven all the way through to measure potential "thread choke" of the frame/barrel junction, thread choke indeed exists and may be a problem. The throat measurements shouldn't have an "extreme spread" of more than .0005", preferably less. Most importantly, the smallest throat should be larger than the smallest dimension of the barrel.

We don't want the boolit to ever encounter a larger dimension than the cylinder throat after it's fired, or it might lose it's seal, get gas-cut from the leaking gas, and lead the barrel. Common problems are cylinder throats smaller than groove diameter, and thread choke. Either of these conditions will probably cause barrel leading and inaccuracy. This isn't "Holy Writ", just my standards based on what I've seen work and not work in my guns, and standards that I correct any new revolver to if it doesn't check out. Works for me.

Hope this sheds some light,

Gear

[mdi]

Gear nailed it. I can't add anything except a refrence;

http://www.lasc.us/Brennan_2-1_MeasuringGunDimensions.htm

[mpmarty]

Ordering a mold without first slugging throats and bore is like ordering rings for an engine without knowing the cylinder diameter or piston size.

[462]

This may be helpful: http://www.lasc.us/Fryxell_Book_Chapter_6_ThroatGroove.htm

[MtGun44]

. . . or like buying shoes based on the size that your brother in law wears.

Could work, but I wouldn't bet on it. Some brands and calibers have a
good record of consistent dimensions, but most are pretty variable, and
some are know to be mostly way off. Best measure.

Bill