Just got a chronograph, shot some Hornady TAP..

75gr TAP (black box) FPD. Humid out. 79 degrees. Elevation probably 40-50ft (Florida)


Colt 6520, 16", 1/7 chrome

Average velocity: 2612 SD: 17



24" stainless, 1/8 upper.

Average velocity: 2716 SD: 30


XM193 in the 24"

Avg: 3356 SD: 54

Any XM193 chrono out of your 16"?

Yes, and it was much hotter than I expected.


Every single source I've read said that 16" barrel should produce 3150 FPS with XM193.


Here's my data:

XM193 55gr, 16" bbl

       3294
3212 AVE:
3268 3249
3246 SD:
3225 33


Averaging 3249 fps. This was NOT from a hot barrel. Hmm...Either there's something about my gun, or this lot of XM is hotter, or the data on the net is wrong by 100fps. Or my chrono is off. My chrono works fine, as every single other ammo was right on the money, from 22 long rifle on up...


Barrel is regular Colt pencil found on a 6520 with 1/7 chrome lined 5.56 chamber.

Interesting!  You only have a ~100fps difference between the two guns.  That's pretty good.

Are the bolts "factory"?  I.E. original Colt bolt in the 16"?  

No.


The 24" rifle is a stainles steel Wilson barrel from M1S. Bolt is not marked, carrier is the "enhanced" one with shrouded firing pin area, and slightly larger rear than regular AR-15 carrier, but no where near as much as the M16 carrier.



The Colt 6520 has the real-deal MPC Colt bolt, with M16 carrier.

Bore materials, rifling style, age of bore, nuances in chamber reaming, chrome lining, and lenght ALL have a significant effect on the muzzle velocity of any ammo.

So does ambient temperature, chamber temp, amount of time round is allowed to sit in chamber before firing, lot to lot variations in the ammo, etc...

Many factors.  I dont find it terribly odd that one of your 16" rifles got 100fps faster than "expected spec".  The ammo lot could determine that alone.