Range Report HSM 77gr FPS Chrono on an 18"???

[11B-B4]

I built a mk12 mod1 copy and was having a cam cut for my leupold mk4 m3. I went with the factory 2680fps numbers but then found that HSM chrono'ed that on a 20" barrel.

Has anyone chrono'ed that ammo through an 18" From what I hear the freebore boost on the 12th model is about ~+15Fps but Im wondering if i should cut back on the muzzle velocity for the cam. Id test it myself but my chrono just took a shit and the order for my cam is in the works. I wanna try to make a correction if need be before it gets cut. Im thinking maybe it would be somewhere around ~2620fps?

Any help would be appreciated.

 

[hitman]

Re: HSM 77gr FPS Chrono on an 18"???

I have cronoed it out of a 16 inch barrel. This was their 5.56 version though not the .223 so it is faster. It cronos tight at 2700 fps out of my Noveskes. So, probably add about another 50 fps for the extra 2 inches. Hope that helps.

 

[11B-B4]

Re: HSM 77gr FPS Chrono on an 18"???

hmm didnt see any HSM 5.56 77grn ammo available. I bought the 77grn 223. Anyway im guessing that I need to chop the velocity 50 fps.

 

[Sapper524]

Re: HSM 77gr FPS Chrono on an 18"???

2838 fps out of my 24 inch Colt CR6724 - 10 shot string - the numbers suprised me but im not complaining.

 

[11B-B4]

Re: HSM 77gr FPS Chrono on an 18"???

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sapper524</div><div class="ubbcode-body">2838 fps out of my 24 inch Colt CR6724 - 10 shot string - the numbers suprised me but im not complaining. </div></div>

Wow thats impressive. I asked a tech guy at HSM about how often if at all do they change powders and he said he doesnt think they had as far back as he can remember so the velocity should stay more often than not the same. He said the spec'd FPS was from a 20" gas gun.