Fast and slow barrels (or is it chambers)?

[Dave King]

On 24Hourcampfire there's a thread having to do with fast and slow barrels (myth or fact essentially). On fella suggested that it may be the chamber in conjunction with the barrel... I got to thinking (yup... a problem as many of you know) about my past experiences with rifles and different chambers reamers. Anyway...

This premise that a chamber has a bit to do with fast and slow had me in a tizzy for a bit (itching to try it) so I ran out and tested some ammo in two different rifles. Here are the results:


The rifles (barrels and chambers)

Rifle 1:

Rem 700 Custom, Match chamber (not tight neck), 24" Mike Rock 1x11.15 barrel.

Rifle 2:

Rem 40X Custom, 7.62 NATO chamber, 26" Hart 1x12 barrel.


The ammunition (same lot fired from both rifles. both barrels pre-fired (warmed))

Ammo 1:

Federal Gold Medal Match 168's (308 Win)

Ammo 2:

Military 7.62MM Long Range M118 (175 Sierra Match King)


Oehler at 6" from muzzle (center screen)

Rifle 1 (24" barrel,match chamber)

GMM 168 average speed 2744fps (StDev=16.8fps)
M118LR average speed 2717fps (StDev = 26fps)

Rifle 2 (26" barrel, 7.62NATO chamber)

GMM 168average speed 2661fps (StDev=19.8fps)
M118LR average speed 2639fps (StDev=19.2fps)


Observed: The rifle with the 24" barrel produced speeds 84 fps faster with the GMM 168s than the rifle with the 26" barrel.

The rifle with the 24" barrel produced speeds 78 fps faster with the M118LR than the rifle with the 26" barrel.

The 40X rifle (26" barrel) showed the faster bullet to be the GMM 168s by 22 fps.

The Custom Rem 700 (24" barrel) showed the faster bullet to be the GMM 168s by 28 fps.


What are the groups thoughts on these data???

[ 10-17-2004: Message edited by: Dave King ]

 

[LB]

I'm not very technical about these things, but I think the only way to attempt to prove ANYTHING is to have something like a Universal receiver, with a screw on bore for two different and interchangeable barrels.

If the chamber is cut with two different reamers, then you could unscrew the rest of the barrel and compare the differences.

One "bore" could record higher velocities with both chambers. On the other hand, "chamber A" might record higher velocities with either "bore"?

Maybe something like this has been done, but comparing a unique barrel with another unique barrel with slightly different dimentions will not prove anything to me, if the purpose is to conclude which aspect records a higher velocity, and reach conclusions.

For now, my opinion is that there are fast and slow barrels, and the above described exercise leaves me unimpressed. (with apologies to Dave)

Good hunting. LB

 

[Dave King]

Not a problem as I spend my time thinking of strange things to do... I was thinking I'd have a barrel match chambered then later have it opened up with the NATO reamer. I could do a end to end comparison with the only change being a chamber size.

 

[LB]

Yes, but maybe the only thing you will have proved is that you moved your max load upward, due to the increased volume, or expansion room. I'm sure that you understand that a constant charge weight produces different velocities, in different barrels. A max load of eightly grains producing 3200 fps in one barrel; may require 80.5 grains, in another barrel, to reach the same maximum pressure and the same velocity. Ten barrels, velocities recorded, and then rechambered with a larger reamer might give you some indications, if every one showed the same improvement. Simple things always get complicated.

 

[Dave King]

LB

Factory loads, that's why it's an interesting puzzle for me. The "known" data for the M118LR is apparently based on 2600fps (24 inch military barrel). That's how this quandry sort of came about, how to quantify drops for tracer ammunition as compared to the "known" M118LR loads. My test rifle with a 24" barrel shows 2718fps but the rifles I'm testing for show 2600fps (M118LR). The tracers show 2833 in my rifle so can I provide useful data??? Now enter the NATO chamber rifle but 26" barrel, not 24 as desired. I can check the 26" NATO chamber rifle against the 24" match rifle with the M118LR but not the tracer (too short on ammo for unnecessary shooting). Anyway, I can somewhat believe that 2" more barrel with a NATO chamber rifle and "factory" ammo will produce the 36fps difference from the 2600 (24" guns) to the 26" I shoot.

It's all just a little diversion for me as well as a puzzle piece.


Just as a note, the NATO ammunition chambers with varying degrees of difficulty in the match chamber... a little snug already (long probably).

 

[Guest]

Dave
It would seem from reading posts here over the last year that the "Rock" barrels are "fast". They seem to routinely clock higher velocities than other makers.
Is this to do with extreme uniformity resulting in a very good gas seal around the projectile and lower internal friction?
I think the barrels you are comparing have more to do with the results than the chambers.
My 02 - APB.

 

[Fiftydriver]

Dave King,

I have seen some variation in the rifles I build similiar to the ones you are seeing.

In your case I would speculate that your custom Rem 700 with its shorter barrel and match chamber is creating more velocity for a couple reasons.

First off, I am sure the throat in the match chamber is much tighter and shorter then the NATO spec chamber. This will create significant velocity over the looser chamber.

Along with that though, the tighter twist on the rifling will also increase load pressure and will result in a bit more velocity on average then a slower twist.

This is only when using the same load, the slower barrel with a topped out load will produce more velocity then the slower twist with max load but in your comparison, the faster twist will get more speed.

Other factors I have seen with velocity variations from one barrel to another is the design of the rifling.

My preferred barrels to use are Lilja barrels. These are built to true BR tight bored specs and as such tend to not produce the extreme velocity that some other brands with looser bores will.

Still with the level of accuracy offered by these barrels, I prefer this over extreme velocity.

Also, different numbers of rifling lands will also effect accuracy. A 3 groove will shoot at a different speed then a 6 or 8 groove.

So while I personally feel the chamber is the largest contributor to velocity levels, there are several others that add up or subtract from velocity as well.

Just what I have been seeing in my rifles.

Good Shooting!!!

Kirby Allen(50)

 

[Mysticplayer]

Dave, interesting tests. If someone wanted to really see differences, this is how I would structure the experiment.

Talk to the barrel maker to make a 40 to 44" barrel. Cut in half to make two barrels of identical finished length. Probably as close to identical as we are likely to get.

Not cut the chambers you want. Test and see what you get with similar chamber pressures. Amount of powder and type of powder burned will make a difference. So reference to a pressure curve and muzzle pressure would also need to be analyzed and compensated for.

What I mean is that if one is a min spec chamber and uses 44gr of Varget compared to a max chamber that uses 46gr of Varget to get the same chamber pressure (same throat and leade dimensions), it would be assumed that the larger volume chamber/case would give the higher vel. Cases would be fireformed.

If using identically sized cases, there will be an error as some energy is spent expanding the small case in the larger chamber.

The vel would have to be adjusted for the difference in powder volume, etc. My guess is that once all numbers are corrected, you will get the same performance within the measuring error. There is no free lunch. Same energy into a system will give the same results.

Repeat but use the same reamer and two different barrels from the same manf. Compare that. I bet that the vel will be different. The pressure data will pretty much tell the tale.

Right now, there are just too many variables to conclude anything.

I believe that fast and slow barrels do exist. However, these barrels are not readily predictable. The same maker will make barrels that vary.

Or else, all barrels would be ...the same.

Jerry

 

[Tailgunner]

Dave
Another variable not often thought of is the throat/freebore area of the chamber.
Had one that was .004 (yep, 4 thou) over bullet diameter, 30-06 26" MV was 2725.
Shortend the barrel 1" and recut the chamber/throat to "min Sammi" and the MV went to 2950 with the same lot of reloads.