Rifle Barrel Muzzle Constriction- (Choke)

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mashburn posted this 30 April 2019

Over the years while cleaning the bore of old Model 700 Remington rifles I would notice that when I got down to about the last 3 inches above the muzzle I would feel more resistance to the cleaning patch. I decided that the Remingtons  that I was dealing with had a smaller bore about the last 3 inches of the bore. I slugged a coule and sure enough there was a restriction where I was feeling the patch tightness. During this time, which was the heyday of my gun building, I had rigged up a shop made version of a bore scope. Those Remington barrels that I had been dealing with gave a big surprise when I could use my scope device and see all of the imperfections in the bore. Most of these bores looked absolutely awful but all of them shot remarkably well. I decided that this was a Remington ploy to make rough barrels shoot well.. So, a gun building buddy of mine and myself started experimenting with this choke idea. Myself, I will not put a junk barrel on a rifle. However, he will build a beautiful gun and stick some of the most gosh awful barrels on them. We made us up a tool so that we could swage the muzzle areas of barrels down. We rigged up a tool similar to a knurling tool only stronger and we put smooth hard wheels on it. We would put this in the cross slide of the lathe and put the barrel in the lathe either between centers or the chamber in in a 4 jaw chuck and the muzzle on a center in the tail stock. This puts a lot of pressure on the tail stock but hasn't effected our lathes. We would then run the rollers up against the muzzle and turn the lathe on at a slow speed (about 50 rpm). We would run the rollers up against the muzzle area and move them back and forth and keep pressure on the tool my keeping pressure against the hand wheel. We learned very soon that you had better stop and slug the barrel because you get results a lot quicker that you expect. If you go too far the only option is to bob it if you have enough barrel to do that.

The first barrel that we did this to had already been fired a bunch and the accuracy was pitiful  After our choking job we couldn't wait to get it back on the gun and fire it. The results were spectacular to what it shot like before.Now , what is your opinion and has anybody done this?

Mashburn

David a. Cogburn

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R. Dupraz posted this 30 April 2019

This is not a new idea. During the heyday of the heavy bench muzzle loading rifles some bores were hand lapped to create a tapered bore, smaller at the muzzle than the breach. Essentially the same thing but from a different direction. Not only supposedly it increased accuracy of the round ball or slug but made the rifle easier to load as well.

And, In 1941 the German army fielded an antitank "rifle" with a tapered , squeeze, bore, to increase velocity and penetration of the shot in order to deal  with the Russian T-34. The 2.8 cm sPzB41 anti tank gun. More commonly known as the Pak 41.

Back when I was building traditional cap lock ML rifles, I used to hand lap the bores from the breach end before the breach plugs were fitted in the Green Mountain barrels that I used at the time. I did hand lap a taper in one of the .50 cal. barrels. It shot well but decided that it took too much time, most of a day, and work so didn't do any others.

Target, 50 cal. heavy Hawken percussion. One inch squares.  Five shots at fifty yards. Lower left a hang fire, Lower right from a  different group.

 

R.  

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John Alexander posted this 30 April 2019

A couple of interesting posts!

Choke or tapered bores isn't talked about much but are pretty common in some rifles.  I have observed "choke" in several high quality air rifles.  I have owned 4 Savage 12 BVSS 223 rifles and when slugged all four had a few tenths of "choke" in the last couple of inches.  Don't know if this was intentional or just a by product of the awful loose place in the middle of each bore stopping just short of the muzzle.  All four would average about .7 moa with good jacketed bullets.  I could never make any of them average less than 1.0 moa with cast and wasted several years trying. My theory - jacketed bullets are elastic and spring back cast bullets aren't and don't.

Another thing in Mashburn's post that is seldom acknowledged is how flawed and rough a lot of barrels are including sometimes big defects right at the muzzle and still outperform some perfect looking barrels. The first two of the Savages mentioned above had bores that in addition to the loose area in the middle looked like the Alaska Range in smoothness but shot jacketed bullet much better than most factory rifles that Am Rifleman tests.

John

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Ross Smith posted this 01 May 2019

Another old blackpowder era idea gaining new interest is the gain twist.

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singleshot posted this 01 May 2019

Harry Pope, Ron Smith (RKS gain twist and choked), and Al Story (BRC- Borchardt Rifle Company) all made/make choked barrels!

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loophole posted this 04 May 2019

I'm surprised that no one has mentioned Calfee's book on rimfire accuracy, which states that a choked barrel is essential to build a 22lr to win matches.

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mashburn posted this 05 May 2019

Hello loophole,

Evidently not many shooters are aware of muzzle constriction. I was just curious after feeling the restriction years ago with a cleaning patch and followed the curiosity up with some measurements and experimentation. The explanation of how a friend and I made the lathe roller tool and tried it on some crappy barrels proved to me that constriction at the muzzle was very beneficial to accuracy. Like I stated, I don't put junk barrels on guns, but if I don't change my mind, the next rifle I build is going to get constricted and I will see what it does on a good barrel.

thanks for your interest and input,

Mashburn

David a. Cogburn

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