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Posted: 8/9/2005 12:48:09 AM EDT
Sorry if this has already been asked, but I did a search and couldn't find the answer.

Does anyone know of a source of information on the ballistics achieved by the 6.5mm Grendel in barrels of different lengths? The normally published figures seem to be from 24 inch barrels, but for military rifle purposes this would be too long; I'm really interested in performance from 20 inch, 16 inch and shorter barrels.

Tony Williams: Military gun and ammunition website and discussion forum
Link Posted: 8/9/2005 5:24:02 PM EDT
I think it's 25 fps per inch. 1911roben.
Link Posted: 8/10/2005 2:56:28 PM EDT
Based on what we have seen to date, the Grendel ballistics from varying barrel lengths run in descreet bands and it is not generally correct to assign a velocity per inch figure when calculating how the cartridge will respond to changes. I have also observed that the lighter bullets are most effected by reduction in barrel length while heavier bullets have a broader range they will operate in before there is a marked change in velocity.

The 123g Scenar generates just over 2650 fps from a 24" barrel, shifting up to 28"adds about 50fps to this value. At the same time the 19" barrel will provide 2610 fps. If you then jump down to a 16" tube the velocity drops to 2530 fps but reducing still further to 13" barely moves the velocity. What is more interesting is that this set of figures is specific to the 123 grain Scenar factory load. If the powder type is changed the bands can be somewhat rearanged, and small changes in bullet construction can have an even bigger influence.

Sorry to ramble but to sum up really briefly I do not have an answer for this question yet.

Bill Alexander
Link Posted: 8/10/2005 4:01:29 PM EDT
Velocity losses will be greater at the short lengths because the pressure loss is greater at those lengths. Once you get up to longer lengths, the pressure delta between 22" and 24" is pretty minor, and that's why velocity gains from going up in barrel length is a diminishing return situation. But when you chop from 16" to 14" or so, that's a significantly greater pressure differential at that point and that's why velocity deltas are also bigger.
Link Posted: 8/11/2005 5:10:23 AM EDT
That is exactly what the theory says should happen, what we are seeing are bands of barrel lengths that have little effect on velocity. When we started working down from the 19" factory barrel the velocity dropped but the graph tailed off much faster than expected so the first thought was that we were into the area of pressure curve which is largely responsible for the work done. What I am not happy about is why the shift from 16" to 13" only moves the muzzle velocity by a few fps, it should drop at at least the same rate. We are obviously retesting for much of the data to eliminate any possible instrument errors and also collecting pressure info for the chamber and muzzle pressures, but to date everthing is repeatable.

I am also examining how the bullet mass effects the shifts in velocity. This is proving a little more difficult as the powder type must be standardized for several bullet masses but we have two candidates XMP5744 and BL(C)-2. The last set up is a set bullet but various powders so the whole program is going to take a a bit to get all the data completed.

Bill Alexander
Link Posted: 8/11/2005 5:35:15 AM EDT
Bill,

Nice to see you postin' over here with the straight dope!!

I'm interested in some of the data you have for 16" barrels. I don't really know the effective velocity to fragmentation/expansion rates for the different bullets/weights......could you shed some light on what kind of range one can expect from a 16" barrel, basically the point at which the projectile retains enough velocity to effectively frag/expand......I know most of the rounds are pretty efficient with high BCs, but what are the actual yard/meter figures??

I'm tryin' to decide on what barrel length to go for to get the most out of the rifle......vis a vis weight and the dimenishing returns of barrel length.

Do you see a 16" barrel in the future line-up?
Link Posted: 8/11/2005 8:11:36 AM EDT

Originally Posted By Trestles:
That is exactly what the theory says should happen, what we are seeing are bands of barrel lengths that have little effect on velocity. When we started working down from the 19" factory barrel the velocity dropped but the graph tailed off much faster than expected so the first thought was that we were into the area of pressure curve which is largely responsible for the work done. What I am not happy about is why the shift from 16" to 13" only moves the muzzle velocity by a few fps, it should drop at at least the same rate. We are obviously retesting for much of the data to eliminate any possible instrument errors and also collecting pressure info for the chamber and muzzle pressures, but to date everthing is repeatable.

I am also examining how the bullet mass effects the shifts in velocity. This is proving a little more difficult as the powder type must be standardized for several bullet masses but we have two candidates XMP5744 and BL(C)-2. The last set up is a set bullet but various powders so the whole program is going to take a a bit to get all the data completed.

Bill Alexander



Does that apply across the board or are you finding that only with powders at each end of the suitable burning range? I would guess that might happen with a quick burning powder, but if you go down in speed to a powder which would provide max velocity at around the 18" - 20" barrel point, as you chop the barrel back more and more you'll see prgressively greater velocity losses, i.e., it won't be linear. That's just a guess on my part though.
Link Posted: 8/11/2005 4:36:59 PM EDT
I am afraid I cannot be a lot of help regarding fragmentation/expansion ranges. Bullet construction is a huge factor in this equation and I am struggling with the number of variables simply finding out how the round behaves as the barrel length varies. I know I need to get to this one but the behaviour might dictate that the bullet is heavier than I expect or the powder type might need to change for a particuler barrel length. I am very much at the start of this now. Bullet construction may also be driving some of the banding of velocities that I am seeing maybe even a bigger inflence than the powder burn rate.

I would also have guessed that the best set up for a longer barrel (heavier bullet, slower powder) would exhibit the least linear behaviour as this is again straight from the theory, but what I currently see is that the lightest bullet (90 grain TNT flat base, Benchmark powder) is the most effected as the barrel length shortens. The heavier bullets (120-150 grain AA2520 BL(C)-2) are the ones that have caused the problems.

The only field data I have for a 16" set up at the moment is from work at Fort Knox. The system was to evaluate the stability of the round over extended range at nominal temperature pressure conditions. Bullets from 90 to 130 grain weight were shot for stability out to 900 yards. I recovered some of the 120 NBT projectiles from the sand bank behind the test gear and these were pretty well blown to pieces. I was also shown some gelatin tests for the 16" barrel again with the 120 NBT. I believe these were shot at 350 yards, I have no measurements from the gel but the penetration depth was given as 18" and the expanded diameter was 0.41" If I remember correctly the retained weight was 52%.

Bill Alexander

Link Posted: 8/12/2005 12:36:57 PM EDT
Thanks for these responses, very illuminating!

Tony Williams: Military gun and ammunition website and discussion forum
Link Posted: 8/12/2005 12:48:13 PM EDT

Originally Posted By Trestles:
Bullets from 90 to 130 grain weight were shot for stability out to 900 yards. I recovered some of the 120 NBT projectiles from the sand bank behind the test gear and these were pretty well blown to pieces. I was also shown some gelatin tests for the 16" barrel again with the 120 NBT. I believe these were shot at 350 yards, I have no measurements from the gel but the penetration depth was given as 18" and the expanded diameter was 0.41" If I remember correctly the retained weight was 52%.

Bill Alexander





What 16" barrel weight/profile have you been experimenting with?

Any guess as to the velocity of the 120NBT at 900 yards.....or even 5-600 for that matter?
Link Posted: 8/12/2005 6:15:19 PM EDT

Originally Posted By TonyWilliams:
Thanks for these responses, very illuminating!




+1
Link Posted: 8/14/2005 7:17:09 AM EDT
The "thing" that was being shot was best described as a test rig, it shot, it cycled, but it certainly was not pretty. The barrel was a 1" diameter bull barrel for stability with a 0.907" gas block, everything was pretty much in the white so that they could add instrumentation as needed.

I was not given the instrument velocities for anything on these tests but if you have a ballistics program you might get close. Set the muzzle velocity at 2500fps and the Bc at 0.458. This is very close for the Nosler Ballistic Tip for the launch velocity. Then just see what it spits out.

Bill Alexander
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