Suggestions for a custom barrel maker (<9in)

[Oso Polaris]

I don't have a shorty 6.5 Grendel and there is a number of reasons that it doesn't make sense. The 300 Blackout was designed for this purpose. There is only so much powder that can be burned in a given length barrel. The bore diameter can affect this rate (large bore allows for larger flame). The next step is to play with powder selection to find a faster burning powder to offset a shorter barrel length. The fastest powders (burn rate) are pistol powders, which makes sense because you're talking about a 3"-5" barrel for most pistols. The most popular powder for short barreled 300BO is H110 (W296 is same powder), and it is basically a pistol powder. With 300BO you are loading 18.5gr of H110 into case with 125gr bullet. My 8" barrel spits it out at 2050fps and 16" barrel at 2250fps, and I believe that somewhere around 14" barrel the bullet achieves maximum velocity (full powder burn). I think that in the 16" barrel the velocity is actually beginning to slow from barrel drag...I definitely see this in my 18" barrel.

You aren't going to outperform the 300BO with shorty 6.5 Grendel. Assuming use of same powder, the smaller bore diameter of 6.5 Grendel will restrict powder consumption compared to .308 bore size of 300BO. Also, case capacity will work against you with 6.5 Grendel forcing you either downloading to roughly 60% of case capacity or loading normal case capacity and throwing +50% of powder (unburnt) into your suppressor for a secondary detonation or create a giant fireball in front of the gun. Finally, 300BO can operate subsonic and run flawlessly using +190gr bullets while the 6.5 Grendel becomes a single shot.

I think the Grendel starts to shine with 16" barrels and longer.