"...Then I shot 5 rounds of the hornady 55-grain v-max load. 5 rounds went into 1.5". That's GREAT accuracy from a worn 14.5" chrome-lined upper, and demonstrates that 1-7" twist service-grade barrels can shoot 55-grain ammo well enough."

Feed the old man what it  wants

I got some more of that 55-grain V-Max load and shot it through the newest AR in the safe, a Denny's Operator 16" midelngth (chrome-lined) barrel with a B&L 2.5-16x50 scope in a LaRue mount:

10 shots, 100 yards from sandbags:



Mean Radius: .33"
Extreme Spread: .93

Sweet!

And that's a 1:7 twist?

That's a chrome-lined midlength gas 1-7" twist barrel.

Sorry - I should have specified that in the first post.

+100 to the OP for taking the time to do this.

1:7 and 55 grain and less projectiles are not "bad" but people to tend to be indiscriminate with their words. "Less good" or "not ideal" are probably better terms.

I'll argue once again that all bullets have ideal RPM ranges. Like your car's wheels and other turning things, there will be speeds where your car vibrates, and other times where it's smooth as butter. The trick with a given bullet is to find the ranges it likes, and make sure they exit the barrel within that range.

Will a FMJ be able to compete with a match bullet? Probably not - one is made to be concentric as a primary design specification, and the other one is made to be less expensive to produce. That's not to say that FMJ's can't be accurate, but does say that in all likelihood they won't be "as consistent most of the time."

Does your rifle have an FF rail?  And is your barrel a GEN 2 barrel with the step cuts to accomodate the OPS INC collar?  It's just a minor difference from the GEN 1 barrels which were simply straight with no step cuts.  When did you buy your barrel/upper?  What other loads have shown a high level of accuracy in your upper?  Thanks.

Gen2 with a LaRue 9.0 rail, assembled this spring. It has given mixed results with other ammo thus far - I've shot some great groups (sub-MOA out past 300 yards) with PRVI 75 grain Match, but I honestly haven't been impressed with how it shoots other heavy loads (SSA 70 and 77 grain 5.56, hornady .223 75 grain match). I bought the barrel new & unfired from another board member (he had bought it this spring) and assembled it myself  (~3 months ago?) with a CMT chrome bolt in a CMT upper.  In a couple of weeks I'm going to try some 52-grain black hills in it.


This barrel has been a great reminder to me of the importance of 10-shot groups. I've shot NUMEROUS loads in it that would put 3 or 5 or more shots into well under a half-minute, but then the rest of the group opened up to 1.5-2.5 MOA. I don't think I breathed for a full minute yesterday after I saw this group had the potential to be sub-moa; I just knew the next shot was going to blow it.

You must have a LTCH (Lip To Charge Handle) hold.  That's some serious eye relief there.

Overspinning is only an issue when the bullet falls apart.  Yes, it CAN reduce the BC of midweight bullets but this is only at very long distances as the bullet axis does not track trajectory.



Now with the lighter bullets in .223 Rem having pressure issues with a 5.56 NATO chamber, this could be due to many factors which cause secondary ignition.  One is a worn bore, the other is the chrome plating offering less starting friction.  It is REAL and can be a serious problem with a badly worn throat.  RSI and their most excellent Pressure Trace hardware/software has shown this, usually with lighter bullets and spherical propellents.  Scary pressures...








Note the initial maximum pressure is well under the SAAMI maximum, this is a sign of something slowing down combustion.  It took a bit of what I call "heat soak" to cause the bulk of the powder to become sensitized to the point of ignition, this is the undesired function of deterrent coatings on the individual granules, a form of either intumescence or ablative effects.  Either way, the granules which have not ignited are now much more sensitive.



This can also happen to ball ammo, the following is presumably Q3131/a.








Again, note the initial maximum is well below SAAMI or the specified maximum for M193.  The barrel is a CM 1:8" with about 3500 rounds down it, this caused the initial bullet friction and ignition pressures to be high but also abrasion of the bullet, making its fit in the latter bore to be imprecise.  This resulted in the bullet having less control over the pressure rise and a premature quench of the cumbustion until the heat soak effect cause remaining propellent to burn much faster than specification.  Or application.  If proof loads are in the 75k range, this load would definitely shuck primers.



For the record, OBT markers are optimum bullet timing marks.  Having the bullet exit at these points is conducive to accuracy.

I shoot a good deal of Berger 30 gr. bullets in 223, 22-250 groundhog hunting. There excellent for the small farms here in N.E. Ohio. A decent 200-225 yard round 223 load, about 275 with a 22-250. Velocity drops off fast. Nothing spectacular in hit's, kinda a small entry, no exit like a 17 Rem.

 They hold up just fine at 4500 fps.+ out of a 22-250 1-12  twist, also very good accuracy out of a RRA. Predator P 1-8.

4198 Is a good powder for these little pills in 223, RL-10 in 22-250.

Nice report OP.  I also have all 1:7 barrels (except for my Larue Stealth SPR which is 1:8) and I shoot a LOT of 55grn stuff and have seen acceptable accuracy at all ranges from 50 yds to 550 yards (500m).  In fact the 1st time I shot at my local club on the LR steel range.... I shot the steel silohettes from 200-500m with a stock LMT Defender 2000 with an Aimpoint and another LMT piston with a DMS-1 1-4x using Privi M193 rounds and I was getting consistant hits even on the 500m steel plates.  It certainly wouldn't win matches by any stretch - but in my experience, 55 grn bullets, even out of mediocre factory ammo with 1:7 barrels are GTG.

Awesome explanation - thanks, Keith!


As a side note, how are OBT's calculated?

Barrel vibrations.  The strain gage measurment system is precise enough to determine barrel vibrations as well as bullet exit.  Timing the exit to when the barrel is "quiet" or at the end of a vibration cycle.

Ok...so the OBT would be different depending on barrel contour, right?

I'm using Varget for my 40 grain Sierra BK 22-250 load. I'm able to squeeze 4050 FPS MV out of my Savage 10 and the terminal performance has been spectacular.



You can see some of the results in here: http://www.ar15.com/forums/topic.html?b=10&f=3&t=604524&page=14





It's always a concern of mine that a round I've sent on it's way will end up somewhere I didn't intend it to. I'd like to say I never miss but that wouldn't be true.



It's helpful to my peace of mind that the varmint style bullets at high velocities will more likely destruct upon impact in the event of a miss.

The only strange thing I see her is someone admitting that their AR doesn't shoot -5.0 MOA groups on an internet forum

With 55gr FMJ-BT reloads, I shoot about 2 MOA and that's good for FMJs.  It shoots better with match ammo, but I'm so damned near sighted that I can't do much better than 1.5 MOA without glass.

Certainly.  Each rifle will be slightly different and OBT can change with temperature, atmospheric pressure etc.

How accurate is the Hornady Tap 55gr training ammo?

And temperature, and humidity, and seating depth, and throat length, and powder charge...

Colt 6920 with Black Hills 55 grain soft point ammo.  The 6920 has a 1/7 twist.



This is only 50 yards shot with an EOTech 512 sight.  I have to find the 100 yard target but it is similar (but the groups are larger).  I'd say 1/7 does just fine with 55 grain ammo.

Well for starters, there's a heck of a lot more velocity with the 40gr bullets vs even 55gr...