Does the OBT table work for AR15 barrels too?

[fatelvis]

Just wondering if the gas block and gas tube mess up the ability of the OBT chart to predict the optimum load for a given AR15. What are your thoughts? Thanks!

 

[CharlieNC]

I think if you adjust the powder burn rate such that the calculated velocity matches the actual velocity, then the calculated barrel time would be useful for OBT.

 

[jelenko]

I've been using GRT for several years with for an AR15 in 223.
After entering the velocities for 7-8 shots [i.e., doing what CharlieNC said], GRT's OBT calculator is usually within +/- .4 grain of the actual nodes; sometimes within .2 grain and, occasionally, right on.

Note:​

Right on' for me is getting groups 1/2 MOA or less.​

With my AR [and over 20 or so different barrels], I've found uncalibrated GRT to overestimate the muzzle velocity by 50fps or so. IIRC, Sierra told me that AR's tend to run slower than bolt actions by ~ 50fps.​

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When I go to develop a new load, because I don't know how close the predicated powder amount is to the actual node, I load up 5 loads .2 gr apart - 2 loads above the GRT predicted charge and 2 below and 1 at the predicted amount.
Since this covers 1 full grain spread, and nodes are 'never' more than 1 gr apart [at least for the size groups I need], you could just pick a charge and do a 5 step ladder.
The reason I use GRT is, after calibrating it with the velocities from the 7-8 shots, to see where the max charge would be and arrange my 5 step ladder so I'm safely within the max.

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[fatelvis]

Thank you so much for the responses guys. Another question I have: does the flash suppressor count as barrel length when figuring OBT? Or is it strictly the length of the barrel without the flash suppressor?

 

[JLT]

Thank you so much for the responses guys. Another question I have: does the flash suppressor count as barrel length when figuring OBT? Or is it strictly the length of the barrel without the flash suppressor?

Muzzle device (flash suppressor, compensator, muzzle brake, brake/tuner, etc.) doesn't count as barrel length. The bullet is free as a bird as soon as the base of the bullet clears the rifling at the crown.