90*F
13.5” Mos Tek 5.56 Mid
Unsuppressed
XM193 Fed
Av 2970
Es 80
Sd 20.6
XM855 Fed
Av 2870
Es 42
Sd 12.9
Norma 69
Av 2651
Es 41
Sd 14
I have no idea why I did it. Probably a combination of boredom, a little extra money in my pocket and finding a vendor that had Criterion Cores in stock.
I assembled a 14.5 and pinned an extended A2. I assembled a 13.9 and pinned a Nox to it and I did a 12.5 that I also run a nox on, when not running direct thread.
In terms of velocity, the 13.9 and 14.5 are about neck and neck with M193 at 3000 fps and M855 at 2900 fps with the 12.5 running about 100 fps slower.
If I had it to do over again, I would have done a 16" for a 10x variable optic, the 14.5 OR 13.9 to run with an ACOG and the 12.5 to run light (red dot and light).
Having the 12.5 and 13.9 almost feels redundant. Standing next to each other without optics, it’s hard to tell them apart, except that the 12.5 is wearing a pistol brace. I actually painted them differently because I was grabbing one, thinking I had the other.
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Running my PLX-C 1-8 on a 12.5 (albiet mine is on Form 2’d machine gun) and a Yankee Hill Turbo K with ASR hub. So far I’m really really liking it and I used my ballistic calculator app to true up my zero to match the M8 Yard reticle as best as I could. I’m getting good first round hits at 500. Its become my GPR.
I was running this all on a 14.5 BCM middy for a long time, but the Suppressor just added to much length and with having a Mk12 style build to run this same suppressor on, I felt there wasn’t much point in using both a 14.5 as a GPR when I had the 18in that the way it was setup wasn’t much heavier albiet not as “handy”. Mk12 is now my go to for when I’m training or doing weekend events with my training group. When its all 6 of us together, I’m more of the dedicated marksman so I run the Mk12. Rest of the time I carry the 12.5 around especially when I’m out doing my rounds on my 300 acres.
The 12.5 is a heck of a lot easier to get on and off the Quad or Side x Side with and still make an accurate shot at Yote’s or other 4 legged pests than the 18in is and with the can on the 12.5 its 1in longer than the 14.5 without a suppressor.
I am running 77gr AAC ammo out of all my rifles now tho. Took to just buying that in bulk every time they run their 10.99/20rds pricing. Up to 6,000 rounds now and at only a marginal increase in price over 55’s. I still use 55’s for general range days as I’ve found inside 200 yards with any of my optics, the 55’s and 77’s have negligable difference in zero shift.
With the 77gr AAC, what velocity did you get between your old 14.5 and your 12.5?
2585 average on the 14.5 and 2525 average with the 12.5
Figured I’d share my experience. I just recently put together a complete upper
13.9 Rosco K9 midlength barrel. BCM receiver/MCMR-13 rail. Microbest bcg, carbine buffer, tubbs spring 4oz buffer.
Chrono data was with 5.56 55gr Frontier. (This stuff is fast)
I shot some Fiocchi 5.56 55gr and it clocked around 2850
Groups were shot using Sig MSR 1-6 with 5.56 77gr AAC, .223 69gr OTM Freedom remans, dirty 223. 55gr Frontier I found leftover on the table at the range.
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Cool pistols and carbines!
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With the Rosco K9 did you have to trim the 13in mcmr or is there enough room to still mount a suppressor?
3140 is in the danger zone.
No but it was very close. I had to pull the rail back off when I went to install the FH because I could not get my wrench on the Nox once I got close to the end of the threads. I think the lengths matched up perfectly for me. I don’t have a suppressor so unable to 100% verify that.
What do you mean by that? I was under impression we wanted to see 3k-3200fps out of 55s??
Out of a 20" rifle, but not a shorter barrel.
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Where’s the sweet spot for 55gr out of a 13.9?
I don’t know that. I’d just be guessing. 2950-3000?
Hornady Frontier is known for sketch pressures. I’ve chronoed a bunch of the 68gr, and every once in a while there’s an outlier that’s like 200fps higher than the normal max range of that loading. Like, faster than M855, which I normally shoot as a constant on a chrono day.
The 55gr has been blamed on the internet for blowing up guns.
Yikes my ignorance is a bit worrisome at times… I was about to buy a case of it. I’ve only chrono’d the Frontier 55s and Fiocchi Range Dynamics 55gr both in 5.56. The Fiocchi was ranging between 2919 and 2980. The 5.56 AAC 77gr OTM out of my 16” Larue was between 2700 and 2760fps with a 5 shot group.
Definitely don’t buy that crap.
Our buddy bought a bunch and we we’re shooting it long range, 750-900 yards. That crap was all over the place. I loaded him some 69gr handloads, and all of the sudden he could track on the targets.
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See that’s strange because I’m having fantastic luck with the AAC 77’s. Took my mk12 to 705 and was clinking away boringly on a 12x20 IPSC
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Good to know. I had a 13.7 at one time and had to shave the “ears” off the front of the 13in MCMR and then it was just fine. I’m debating on switching to a 13.9 for my GPR vs 12.5 and the Rosco K9 is at the top of the list for barrel.
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Not sure if you’re replying about my post, but AAC’s current stuff is probably ok, but the Frontier is what we were not loving.