Okay, after extensive reading on the matter on this board and on TF I know that one should only go up to 69 gr loads out of 1x9 barrels.  So, I was a bit surprised when I contacted a PROMINENT Colt parts and rifle distributor looking for a 1x7, 16" lightweight A4 upper, but Colt doesn't make one anymore, I guess.  We then started talking about the new heavier 5.56mm loads and about rifle twists.  And that's when their service rep stated that 1x9 barrels do just fine in shooting 75 gr bullets.  Am I missing something here?

My 1/9" bbl stabilizes the 75 grain Hornady fine.

It blows up the 45 grain SP bullets, though.

Just my opinion, but it may depend on how far you want it to stabalize.  But just so you know, the hornady website shows their 75 grain bthp for 1/9 barrels.  But some here say that as the range extends it may not stabalize.  At least I think that's what I read here.  I have a 1/7 so my words are based on what I think I've read here and other places, not on experience.

I am going to take off my 16" 1x9 twist chromoly Wilson M4 barrel and install a new
DPMS mil-spec 16" chrome-lined M4 barrel with
a 1x7 twist and one of the new .223 Ops Inc. brakes, to handle the new bad boy 77g Black Hills.  At 200 yards or farther, the 1x9 just does not cut it.  The Black Hill's red box 75's grouped great at 100 yards ( even better that the LC Federal XM 193 , but went all over the place at 200, and 300 yards.  If you are seriously wanting to use the 75's or 77's, I do think that you should opt for the 1x7 twist barrel.
CMMG has them in stock....

[url]http://www.cmmginc.com/inventory_a/Barrels_tbl.html[/url]

I've fire 75gr Hornady OTM, 77gr SKM, and 77gr Nosler (MK262 Mod1) from 3 different 1:9 twist barrels:  4 year old, 8K rounds of use 14.5", 2 year old 2-3K rounds of use 20", and few month old less than 500 rounds of use 16".

All three were from Bushmaster and were chrome lined.

All three performed adequately at 100 yards (met M16 performance spec).  I've done some testing at 200 yards with the 14.5" and it was adequate.

I should also note the boxes of Black Hills 75gr I had were marked "For use in 1:9 twist or faster".