My question is just basic performance and distance of the 545 and if it suffers the same performance degradation out of the shorter barrel that the 556 does. Being that the 545 in say 7n6 has the air pocket in front of the round does it require a certain speed to deforms and yaw like the 556 does. I know the 556 will still perform as it should but with a shorter barrel those distances are shorter than in a 16-20" barrel as opposed to a 10-11.5" one.

If there is a resource for that my apologies I don't know it.

I can attest to the fact that 5.45 does not like short barrels.

I'll more then likely catch flak for this but I'd go 5.56. both rounds rely on velocity to fragment, so out of short barrels they both become ice picks with surplus ammo(especially at intermediate ranges). the reason I picked 5.56 is because in the US, we have access to a plethora of different bullets that are specifically designed to still fragment at lower velocity (ex: Barns TSX, Federal fusion, MK 262). I'll admit that still doesn't stop me from wanting a 16'' SLR-104, but for SBR's I'll stick to 5.56 and 7.62x39.

The DoD uses a 14.5" bbl with the M-4 for general issue.  There are numerous units that also employ the weapon in a 10.5" configuration, to include units in the UK and GER. HK manufactures the 416 in a 10.5" version. My point is that folks that use these weapons in combat, to kill people, are comfortable with the 10.5" config.  It is a trade off but still effective.  I caveat that I'm no ballistics expert and I'm well aware that guys were bringing uppers into the box that they probably had no idea how'd they'd perform. What I'm commenting on is the actual units that have issued weapons available in the 10.5" bbl config. Prior to those weapons being made available for issue there is extensive testing done at the national armories and depots.

As for the 5.45 I've had trouble getting the rounds to stabilize in a 10.5" bbl. I currently have a SBR that shoots well with 7N6 but I swapped 3 bbls out before I found  one that the rounds wouldn't tumble at 25 m.

What barrels did you try?   Was this in an AK or ar?  I have a 10.5 ballistic advantage on my 5.45 sbr ar-15.  I usually shoot it at 100 yards, haven't had an issue with not stabilizing or tumbling

What's the twist rate on that BA barrel?

I've got a 16" AR upper in 5.45x39 I've been thinking of having cut down to 10" but it's a 1in9 twist and I've yet to find anyone that's running that. 1/8 seems to be most common for US made 16" barrels, and for a 10" barrel I'd think you'd want something tighter than 1/9.

But I'm not sure what an AK-74's twist rate is, I've read 1/7, 1/8 and 1/9.

The 5.45's primary wounding mechanism is tumbling inside the target.  So I imagine that it's less affected by going to an sbr.

5.45 performs great in short combloc barrels. Maybe the American barrels are not the correct twist rate. I have an AKSU SBR that has never had a key hole or any issue, and it's an 8" barrel. Velocity loss is also not a huge deal with 5.45 or 7.62. 556 is more barrel dependent than 5.45, but there are way more options in 556 than 5.45.

Mrgunsngear has a great video showing what 7N6 can do from a Krinkov sized weapon.

5.45x39 7N6 Ballistics Gel Test With A SBR'd Krink (HD)

I dont remember.  And the whole barrel is covered by the rail.