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Posted: 1/14/2015 4:16:22 AM EST
Gunsmiths and engineers,
I am trying to decide on barrel length for a 6.5/.280 variant gas gun upper to shoot at long range. In discussing this with the gunsmith, he indicated that there are two big variables to understand: first, barrel "whip" with the long barrels and second, the bullet should exit the barrel before the bolt begins to move as this could also affect accuracy. I'm trying to understand this, so this brings up some questions.

What is the rate that the pressure wave moves? Or, in a regular rifle length gas system, how long does it take for the bolt to begin unlocking after the gas reaches the gas port?

Is there some relationship between the length of the gas system and the amount of free barrel beyond the gas port that ensures that the bullet leaves the end of the barrel before the bolt begins to unlock? Since there is a range of bullet speeds, I assume this would have to be the based on the fastest bullet I anticipate using. Since this is LR, more than likely, I will use 140-ish grain bullets.

Also, I understand that there are different length gas systems but don't anything about them. For instance, is there a relationship between gas system length and gas port size? Is there a relationship between length and gas tube diameter? This will be a "large frame" style AR so it should use the large diameter gas tube, I assume, unless something else dictates the use of the smaller diameter tube?

Many of the very reputable custom builders will not cut a barrel more than 22", but that certainly is not optimum for bullet speed. Maybe they know something?

Just trying to understand - halp

Harry
Link Posted: 1/14/2015 8:51:30 AM EST
[Last Edit: 1/14/2015 9:09:35 AM EST by KILLERB6]
Just to kick this off:

1. I don't know of any lock time data for the cartridge you plan on shooting, but it may be out there. I will post a link to what I have for 5.56 for reference if I can find it.

GRAPH 1.

GRAPH 2.

[Again, for reference; you would have to find/compute something similar for your cartridge]

2. As for barrel harmonics: there are things you can do (like fluting) which affect harmonics, but it can't be eliminated. As long as the whip is consistent, bullet impact should be consistent (all else equal).

3. The other (some say only) factor to consider (barring an operational consideration like as short as possible so you can run a suppressor without having a 6' long rifle) when choosing barrel length is long enough to ensure that your projectile remains sonic at the longest range at which you intend to engage targets (i.e. doesn't drop below sonic prior to target impact).

4. As for barrel length versus velocity, there have been a couple of "studies" where they progressively cut an inch off of a barrel and the bottomline is: barrel length really doesn't affect velocity that much. I will try to find/post the LINK. There are others, but this one is representative of the relationship.

ETA:

5. Forgot to mention that for a precision gun I would go with a rifle-length gas system.
Link Posted: 1/14/2015 10:44:14 AM EST
The USGI M16A1 has a recorded dwell time of roughly 10-11 milliseconds.

Dwell as defined in Addendum to Final Report on Product Improvement Test of Redesigned Buffer for M16A1 is a measurement from the point of initial bolt carrier contact with the barrel extension, at conclusion of counter recoil, to initial rearward movement of the carrier as next round fires.
Link Posted: 1/14/2015 1:25:17 PM EST
Contact Scott Medesha - Medesha Firearms. He has experience building 6.5/.260 AR's for long range competition and can suggest gas port size and location probably better than anyone else.
Link Posted: 1/14/2015 6:45:32 PM EST
Thank y'all for your input. I appreciate it very much, thanks for taking the time. Anybody know anything 'bout this: http://the-long-family.com/? This is a concept called Optimum Barrel Time.

Harry
Link Posted: 1/16/2015 8:30:41 PM EST
As far as all of those issues go,
Barrel Whip - if you were talking about a bolt rifle with a free floated barrel, this would probably be more of a valid/issue or concern. However if you are talking about a semi-auto, the other things attached to the barrel (gas block, gas tube) will also impact the barrel behavior. I am not aware of anyone who has tried, or been able, to "model" this behavior to a point that they were building rifles based on that information.

Bullet Exit & Bolt Movement - not sure if you have read this:
http://ar15barrels.com/prod/operation.shtml

Hate to say it, and don't take it the wrong way, but are you pretty much trying to "reinvent the wheel" or trying to build an "enigma wrapped in a conundrum"?

Grab whatever barrel length that works best for your application,
Short - better handling, positional shooting, closer ranges
Longer - bench or prone shooting, longer ranges

Go with the longest gas system that will work with your barrel length. Longer gas systems operate smoother/better.

Then build/tune a handload that works optimally with that setup.

By tweaking your pressure & velocity in your load, you will be able to match that with your barrel and gas system performance for optimal results. This is the "way that business is done".

Best of Luck,
M Richardson
Link Posted: 1/18/2015 6:51:47 AM EST
Thanks! After going round and round with this - no pun intended - I have come to your conclusion. What ever barrel length, then tune the load to the barrel. Thanks very much for your help.
Harry
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