Muzzle Threading — Don’t Remove Too Much Steel
Our friend Robert Whitley of ARX Enterprises LLC has learned, through careful measurement and testing, that some barrels threaded 5/8″ x 24 tpi at the muzzle may not deliver optimal accuracy. The reason is that the end of the barrel can bell out slightly, like a trombone, because too much steel has been removed. This is particularly true with .30-caliber barrels, but it can also be a problem with smaller caliber barrels (even 6mm). Robert demonstrates this phenomenon in the video below. All gunsmiths, and anyone considering threading a barrel, should watch the video. At 1:00 – 1:30 Robert gauges a 5/8″ x 24-threaded .30-Caliber barrel. You can see the belling effect clear as day.
“When setting up a commercial barrel in the lathe, we noticed that the maximum-sized bushing that would fit in the bore at the chamber end was almost .0015” smaller [than what would fit] at the muzzle. That precipitated my pin-gauging of a number of different commercial barrels that were threaded for 5/8” x 24 tpi. What I found is what’s shown on the video.” – R. Whitley
Solve Problem with a Larger Thread Diameter
If 5/8″ x 24 threading is potentially harmful to accuracy, is there a solution? Yes, you simply need to leave a little more steel on the barrel. (See Video starting at 02:40.) Frank Green of Bartlein barrels states: “We get these questions all the time. I say run the largest thread diameter that is possible.” Robert Whitley has found that a 3/4″ x 28 tpi threading does not cause the “belling effect”. Accordingly Robert recommends 3/4″ x 28 if you need to thread your barrel for a muzzle brake or suppressor. Robert explains: “We only make 3/4” x 28 tpi muzzle brakes and that’s what we recommend to customers.”
“See how much meatier the 3/4″ threading is vs. the 5/8″. The 3/4″ threading offers a lot more metal around the bore. There’s a lot less opportunity for the bore to become bell-mouthed…” – Robert Whitley
Barrel Threading Diameter — What’s Important to Know
By Robert Whitley
In truth, the 5/8” x 24 tpi threading never came out of any accuracy-based think tank or set-up, it’s a military .30-Cal threading for barrels that someone has to carry around (they needed to keep the barrel weight down so it was smaller in diameter and the threading had to work with that situation). People have somehow assumed because the military uses that threading for certain things that it must mean that it’s also fine for a highly accurate rifle too, but that’s not really correct.
I don’t think there is any better and realistic option than the 3/4” muzzle threading, and we also do it so there is no relief cut behind the threads on the barrel (i.e. put the relief cut on the brake or jam nut, don’t chop down on the muzzle of the barrel). For some reason many have a hard time grasping that the metal at the muzzle end of a rifle is “sacred” and you should not cut it down any more than absolutely necessary. A little threaded pencil diameter nub on the end of a barrel is not ideal for accuracy especially if it’s threaded and you need to torque on it. I cringe when I see a barrel with something like an MTU or Heavy Varmint contour, only to have an itty-bitty pencil thin threaded nub right at the muzzle so someone can “screw on a can” or a muzzle brake.
Lessons Learned Over the Years
A number of years ago I did a 30BR rifle project with Craig Kostyshyn who was big in the 30BR game and he made some of the best 30BR rifle barrels for benchrest competition. When I did the project I wanted a medium-heavy Palma type contour barrel I could use and also have a muzzle turndown for a front sight band. When he found out I was going to have the muzzle turned down he said “whoa, I need to provide for that when I make the barrel because if you turn the front down later you’ll be shooting a trombone” (i.e. the muzzle bore dimension would open up).
What he did was rough contour the barrel with the turndown (about .010” oversize) before he lapped the barrel, then when he lapped the barrel he took it easy in the muzzle area and worked the back of the barrel more. I thought he was a little bit excessive in his concerns but the barrel shot great and I wasn’t going to argue with him, after all he was shooting groups in the ones. I kind of just filed that away and never thought about it until recently when I went to have Fred from Sabreco do some chamber re-work on a commercial .30-caliber barrel I had. When setting up the barrel in the lathe and indicating things Fred noticed that the maximum-sized bushing that would fit in the bore at the chamber end was almost .0015” smaller [than what would fit] at the muzzle and he mentioned it to me. That precipitated my pin- gauging of a number of different commercial barrels I had that were threaded for 5/8” x 24 tpi. What I found is what’s shown on the video.
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Tags: ARX Enterprises, Barrel, Barrel Threading, Muzzle Brake, Robert Whitley, Suppressor
The barrel could be bored out to just past the threaded portion to maintain accuracy.
So the virtual industry standard 5/8-24 (or smaller) for the vast, vast majority of brakes in the marketplace today is wrong? And those folks having barrels with a muzzle too thin to support a 3/4 x anything thread are sorry out of luck? Or are we really seeing wrong tooling and/or feeds and speeds in the threaded section machining? Buyer be aware.
One thing that is extremely relevant to this discussion is the difference between button, and cut rifled barrels. Even though many (and all premium, that I am aware of) button barrel makers stress relieve their barrels after they are rifled, before they are contoured, there is a small degree of bore dimension change when they are contoured, that is taken care of in the final lapping, if they are lapped. This expansion is the result of residual stress from the buttoning operation, that is not present in cut barrels such as Krieger, Bartlein, and Brux.
I suspect that the issue of “belling” has been recognized for a while now.
Let’s go WAY back to Lee Enfield barrels.
They were NOT buttoned or gang-broached, but cut with MANY passes of a traditional single-tooth cutter.
But it gets more interesting. The basic “blank” was a “forging” that started out as a bar of steel not much bigger than final diameter. The breech end was formed on this billet by heating and “upsetting”. (No, not being rude to it), but by heating it up in a forge and literally hammering /dropping it on that end to make it bulge out to form the bulk needed at the breech reinforce. This probably also had the useful side effect of “rearranging” the crystal / grain structure in this area to provide better resistance to the radial shock of firing.
Once the bar was roughly formed it was checked for straightness, adjusted, stress relieved etc, before being drilled and reamed.
I have several MINT barrels for the No1 (SMLE) rifle here and they ALL show some “belling” of the lands at the muzzle. I strongly suspect that this is a result of the final lapping.
Having said that, VERY early SMLE barrels sere TAPER lapped. Why? they were attempting to approach the velocities of the earlier “long” Lee Enfield barrels by reducing the height of the lands as they approached the muzzle. (This with the 220gn, hard nicked-jacketed Mk6 bullet as well!
The peculiar thing about Lee Enfield barrels, apart from the 5-groove, left-hand twist, is the dimensions of the rifling. The “bore” size was very tightly specified at: 0.301″Min / 0.304″Max. HOWEVER, groove depth runs from 0.005″ to 0.008″. Thus you could end up with a maximum “acceptable” groove diameter of 0.320″! This with a 0.311″ diameter bullet.
The past is a very foreign country, indeed.
So, the British .303 cartridge was essentially that very “modern” thing, a “bore-rider”.
The advent of the “high-velocity” Mk7 spitzer bullet threw all of the previous practices out the window and from about 1913 onwards, all barrels were parallel bored and rifled. The only problem was the muzzle belling resulting from over-zealous final lapping that continued to the very end of production.
I believe that I witnessed this effect on my AR-15 when testing accuracy with and without the flash hider attached.
Removing the flash hider resulted in bullets printing an inch low at 100 yds. Replacing the hider brought them back on target.
I never could figure out why until I saw this video.
Hank. What you are experiencing is more likley due to the flash hider acting like a tuner or muzzle device than a “choke” bringing the bore back to dimension.
Are the barrels in the video all button rifled?
The conventional wisdom is that button barrel bores expand when you contour them, hammer forged bores actually shrink, and cut rifled stay the same.
Barry, From what I understand only two of the barrels were cut rifled and were ours and also these were the only ones where the muzzle didn’t go open up at all. The rest were button from what I understand. Maybe Robert could chime in here and explain it in a little more detail.
Even just cutting and crowning the muzzle if you hit a residual stress point the bore can open up. This is also a reason I would never flute a button rifled barrel as well. Residual stress. No barrel maker can measure for it and it’s more prominent or problematic in a button rifled barrel.
Later, Frank
Bartlein Barrels
Muzzle brakes and flash hiders even if there is no stress in the blank you can over tighten them and effect the bore dimensions and cause accuracy problems. Be careful how you install these as well.
Later, Frank
I couldn’t tell you how many cut rifled barrels I installed brakes on with tennon dia. from 1/2″x28 up to 1″x24 The fact that I offered a 1/2 moa guarantee and free barrel replacement if it did not meet that standard on rifles I built would insure I would get a call and a rework of that rifle. In 6 years I replaced 3 barrels and only 1 had a brake. So if in fact bore expansion does occur you must also quantify the results of this occurrence in 5/8 or smaller tennon dia.
I’m in the process of having a rifle rebarreled, and the selection of the muzzle brake is my choice. This information further defines my choice, so I appreciate it. Thank you.
Question – What’s that collar installed on the barrel with 3/4″x28 threads? Is that for muzzle attachments to tighten against?
If I’m planning to turn down a .809″ barrel for 3/4″ threads instead of 5/8″ – Do you think I’ll need to install something like this collar? Thanks for your advice.
A more interesting study would consider the effect on precision.