According to the Speer manual and their 120gr Gold Dot bullet, velocity loss from 24" length is 20fps deducted for each inch shorter. So we're talking ballpark only, but 8in shorter should equate to around 150-170fps velocity loss.
I like the Federal Fusion MSR ammunition line, and have found the 62gr 223 version to be both accurate and hard hitting.
IMHO the bullet probably come from Speer (same manufacturing group) and the construction very closely parallels the Gold Dot.
I would suggest their
6.5G 120gr load. Claiming 2600fps, I'd assume a 24" test barrel, and suggest 2450fps as a more realistic speed out of a 16" barrel.
Just a SWAG, I'd be pretty comfortable with the estimate.
I'm in the Corona-Derailed process of load development testing of the 6.5g Gold Dot in a pair of AR's, 24" and 20". It's tricky, because I suspect the AR 6.5G ejector is not strong enough to permit full bore loads in a 24" barrel (longer dwell accentuating ejector stress), but there's a bright side.
Bolt guns don't have such problems; and I have already accepted the 62gr 223 Fusion MSR load for my 20" Mossberg MVP Predator bolt gun. It becomes a deer rifle at distances up to around 150-200yd. What's sauce for the bolt gun is often not sauce for the AR; but the bolt gun comes out on top with this adage. I live In SE AZ now, but a 200yd Deer shot back in my old digs (Central NY Finger Lakes District) was an almost impossibly long shot to find.
I have a Savage 10FCM Scout with a 7.62x39 chambering. I had bought it with the intention of swapping out the barrel for one of the PPC iterations. It shot so good, I abandoned that idea.
But now that 6.5g barrels are available, maybe the plan could work with that modification. It's a plan long in the making; but it's also evolving nicely.
Have you tried looking at
these? $1050 list, probably cheaper
from a dealer, or at auction; add optics, sling and bipod, and you're off to the races. The 6.5 Creed can be your Huckleberry; or the 223 can reach to 600 reliably, pretty Target-Ok out to 800 with the right loads, and is 33.5" long, 7.5lb...
Greg