deeply recessed crown question

B

backbencher

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Gentlemen, et al:

This is not strictly a benchrest question, but I was sent to you as the most knowledgeable forum about escaping gas disruption on bullet accuracy.

Givens - bullet: .355" 124 grn FMJ, BC .166 G1
MV: 1050 fps @ the true muzzle
Crown depth: 1.7"
Crown dia: .472"
Barrel length to true muzzle: 3"
Total length of barrel: 4.7"
Chamber pressure: 36,550 psi

If the crown is bored true, all else being equal, how bad will the disruption be to the bullet as it passes through the 1.7" recessed crown?

What I am interested in is firing US M882 9x19mm NATO 124 grn FMJ ammunition subsonic from the Beretta 92, in order to avoid the transonic transition, which in hot temperatures, can occur as short as 15 yards from the muzzle. Based on Winchester's data for their NATO load which they publish based on a 4" barrel, and Ballistics by the Inch's measurements for 124 grn loads, I estimate a 3" barrel to give a subsonic MV. Unfortunately, the shortest barrel that can be securely locked into a standard Beretta 92 slide is 4.7". The rather thin barrel can be bored out from the muzzle; however, this leaves very little room for gas to escape past the bullet, and I turn to you gentlemen, et al, to tell me if such a modification will utterly destroy the accuracy of the barrel, or simply slow the bullet down.

The simplicity of muzzle brake vents cut in the barrel past the 3" mark was considered and rejected due to the requirement of night vision.

If requested and allowed, I will post a link to the discussion on a non-benchrest forum. Thank you for your time.
 
I believe it will destroy accuracy and NOT materially slow the bullet down.

You're trying to slow the car down by spinning it's tires and IMO you're gonna' get too much traction even with the bore opened up, for two reasons.... #1, 50 thou clearance isn't enough to begin with and #2 the bullet will obturate somewhat.

I'm just playing this off the cuff, SWAGging as it were.

al
 
To this question only:

If the crown is bored true, all else being equal, how bad will the disruption be to the bullet as it passes through the 1.7" recessed crown?
Click to expand...

As far as I know, we do not know the answer. It sounds like you guys are familiar with Harold Vaughn's work (Rifle Accuracy Facts). He devoted a whole chapter to the issue of the effect of gas exiting the bore on the bullet, and was not able to come up with definitive (i.e., predictive) answers on what caused non-uniform gas dispersion, or how great the effect was in terms of dispersion.

Given this dispersion does happen -- though not always -- you are statistically better off the lower the gas pressure at the muzzle. What you're proposing is pretty slight.

Best, probably, would be to bleed off the gas while the bullet is still in the barrel, with the vents being in the barrel groves only. Given your problem at night, you would then need to thread the barrel to take a solid tube to cover the holes, where the tube i.d. is what, a quarter inch larger than the barrel o.d.? Some experimentation should answer that, and I'd bet the offending part of the muzzle flash would be gone.

Something along those lines.

Here's a different question: Try what you propose as the simple solution. To avoid argument, grant there will be some increased dispersion of the bullet. Is that increased dispersion significant given your purposes? For example, suppose it increases dispersion .2 MOA over 10 shots. Does that matter?
 
In the book The Bullets Flight by F.W. Mann he had Pope build a barrel with 8 rows of vent holes entering the bore between the lands fitted with machine screws. Extensive testing with all vents open or a combination of vents opened and closed did not provide any noticeable improvement in accuracy.

J.Louis
 
Thank you for your consideration, gentlemen. To the 1st point, yes, I'm afraid that the limited reaming we can do might not be enough. My 3" number is a reasonable guess, but would have to be confirmed by experimentation - perhaps a 1/2" at a time. We may find out, say, @ the 4" mark, that the increased pressure in front of the bullet slows it to subsonic, without greatly effecting accuracy. Or, we might have to bore out all the way to the 2" mark. Given the thinness of the barrel (restricted by the existing slide), and our intention of threading the muzzle for a conical accuracy bushing, we can't ream very deeply.

To the point about a flash hider - what I wish to avoid is making the flash from the Beretta worse than it already is. A barrel vented upwards, above the slide, would make things worse - a back bored barrel would certainly improve the existing situation, acting as an old-fashioned cone flashhider.

To the last question - assuming increased dispersion of the bullet, if the dispersion in the back bored portion is less than the dispersion of a supersonic bullet dropping below the speed of sound, then we will have a net accuracy gain @ 50 m. Depending on temperature, M882 drops subsonic between the 10 & 30 m mark.

Sadly, due to the construction of the Beretta 92, only the top portion of the barrel is exposed, thus we can't sleeve the barrel. I proposed machining a custom barrel from stock, with an expansion chamber on the top of barrel, which would solve the problem, but entirely custom barrels machined from scratch are a bit rich for my blood at the moment.
 
This is just an off the cuff idea. But you could go the direction of the 96g conversion to 40 cal as far as the OD of the exterior tube around the 9mm barrel. So instead of reducing the wall thickness from the inside by reaming, you could contour it down from the outside and slip a thin wall exterior tube over it that would fit the 96G slide. You will not have a lot of free space between the two, but probably as much as you would around the bullet and the back bored section. This would bleed the pressure from closer to the chamber - reducing the MV, without introducing additional turbulence of high velocity gas passing the projectile in a confined space. And the 96G slide and 9mm barrel are fairly available non-custom items.
 
Vibe, that's something we've not thought of - I'll ask the Beretta folks if a 96 slide will fit on a 92 frame. We were thinking of swapping the slide anyway in order to fit the barrel bushing, and there are 9mm conversion barrels for the 96 slide. I am stuck w/ the lower, however, just as I'm stuck w/ the M882 124 grn ammo. And we know where to get a tube w/ the outside diameter we need - 1 sacrificial .40" barrel...
 
I'd seen the conversion kit from CTD, but they claim they are no longer available from them.
http://www.cheaperthandirt.com/product/GNS-049
Beretta Model 96 Complete Slide Assembly Complete slide with sights, barrel, all parts and springs, attach to your Model 92 9mm and convert your 9mm to .40-S&W.
Click to expand...

Perhaps someone else still has one (or some) in stock.
 
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Ok, a cross post from the thread on the other forum:

Where & how do we vent the barrel?

After we turn down the 9mm bbl, & bore out the .40", is there enough metal to enclose the pressure in the 9mm bbl, & will the .40" bbl erode internally from dumping supersonic gas 1mm? away from the wall?

Are we inadvertently constructing a silencer, or just a very complex flash hider?

Is there any accuracy gain @ 50 yds over a normal match bbl?

1 - We have eliminated the transonic transition @ the 10-30 m mark.
2 - The barrel bushing is well tried, & accuracy can be compared between a match 9mm bbl w/ bushing & the freefloat tube.
3 - As we have turned down the bbl, it is less stiff than a match bbl.
4 - As we have a barrel bushing @ the muzzle of the free float tube, we have the bbl solidly locked to the slide, & simultaneously free floated.
5 - We are venting gas from the bbl into the tube channel, lowering chamber pressure, velocity, and dumping gas @ the muzzle. We may get around the muzzle issue by having the 9mm bbl @ the stock 4.9" & the 40" float tube @ 4.7", the front of the slide.

A drawing of the current idea:

.40" S&W bbl wall
__________________________________________________ __4.7"
screw interface w/ 9mm bbl
__________________________Vent____________________ ________4.9"
9mm bore
__________________________Vent____________________ ________4.9"
screw interface w/ 9mm bbl
__________________________________________________ ___4.7"
.40"S&W bbl wall
 
And the sleeving/free float idea is a dead end for 9mm in the Beretta 92. The 96 .40" bbl has the same external dimensions as the 92 9mm bbl, so I don't see how it's possible to safely sleeve a barrel & maintain structural integrity. Next year, I would be interested in funding an experiment with a progressively back-bored 92 barrel, using a Ransom rest, a chronograph, a 50m range, & civilian M882 spec ammo, which I know is available from Winchester, @ least. If anyone would be interested in participating, or loaning equipment, please drop me a line - I'll post again when steady-state scratch is generated.
 
If you bleed gas off behind the bullet into an expansion area, you have just created a silencer/suppressor so beware!!! But why don't you go that way to begin with?

Hovis
 
Hovis, that was a concern. If we just redirect it, it would just be a flash hider or muzzle break. However, if we just back bore the bbl, we're creating an odd type of target crown, or a cylindrical flash suppressor, but not a suppressor.

If we just screw a suppressor to the muzzle of a Beretta 92 throwing 124 grn bullets supersonically, we quiet the report, but do nothing to reduce the dispersion of the bullet caused by the transition of the bullet to subsonic speeds. That transition occurs around the 30 m mark from the muzzle of the 92; around the 10m mark in high temperatures with M882 ammunition.

I've just come into a bit of unexpected scratch - not steady state, but if someone wanted to play with this, I could pay for ammo & a barrel. I could also pay for a reasonable amount of machine time, if the rates are reasonable. You'd need a Ransom rest, a chrono, & a 50m range. And a Beretta 92/96/M9 of course.
 
The purpose of this ridiculously deep crown is to limit the MV to a subsonic speed, thus eliminating the dispersion from the transonic transition. This particular problem exists with the Beretta 92 & M882 124 grn FMJ service ammunition. If there is not a significant dispersion from the deep crown, this would translate into increased accuracy around the 50m mark. This problem does not exist with appropriately matched ammunition to the barrel length - ie, 147 grn bullets leave the Beretta barrel subsonic, & do not suffer the transition. However, given M882 124 grn FMJ ammo, the problem does exist. What is not known is if the proposed cure is worse than the disease.
 
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