Jumping bullets in AR

[Viperdoc]

From mag length in my AR, my Sierra 69 MK bullets jump around 0.037 with a SAAMI min chamber.

I was thinking for my next barrel going with a zero freebore reamer, which is 0.025 less than SAMMI. I could then set the bullets say 0.013-016 or so deeper and then could shoot mag length loads that are either just touching or short of the lands.

The advantages would be a lot less bullet jump, and it would give me more room for barrel wear while still shooting at mag length, so more accuracy for longer.

The extra seating depth of 0.015 or so should not be too limiting on powder volume, I think.

Is this logical or am I missing something?

 

[fdshuster]

Would work fine with the lighter/shorter bullets, maybe even the 69 gr. SMK #1380's. Heavier, like the 77 & 80 grain would really intrude on case powder capacity. When I spec out a new chambering reamer (PTG), throat depth is my first consideration, for the same reasons you mentioned. One of my 223 Rem. reamers has a freebore of .074" and it works fine with all bullet weights from the 55 V-Max to the 80 gr. SMK, but if I wanted to limit my choices to the 69 SMK, or lighter, I too would have chosen a zero FB. Excess FB is no different than firing a couple thousand rounds down the barrel.

 

[Viperdoc]

Thanks for the input. Bigger question is will it make a difference in accuracy in an AR? Since it's a 9 twist my goal is to shoot varmint bullets 50-64gr and Sierra 53 and 69 MK primarily.

 

[fdshuster]

I've never seen any dis-advantages with minimum FB, and you will pickup a little more in velocity when they are seated to touch or very close. Don't know if you're into competition like NRA High Power or the CMP match's, but if you are, I would never seat them to jam. If you have a round chambered & they call to make the line safe, for whatever reason, your chances of sticking a bullet in the barrel, and dumping powder into the receiver will greatly increase. To me, just not worth the risk. I usually seat them to just lightly touch or ,020" of jump, whichever works best for each of my 3 uppers. Will it make any difference in accuracy? Who knows, 'til you try it. My 223's are very forgiving about seating depth/ anything reasonable works.

 

[NorCalMikie]

And, something else to consider. Set the OAL on a few rounds at a jam into the lands. Single feed and see what it does for accuracy. You rig may prefer a jump rather than a jam. Won't know till you try. And it sure beats spending $$ on a barrel modification if it ends up not making any difference.

 

[Viperdoc]

Great points all. I have gotten to know the Krieger guys personally, since they are only 15 minutes away. Right now I use all Lapua brass, FL resized, run through mandrel and then neck turned, chamfered and then loaded. Pockets uniformed.

In other words, BR methods for an AR. ( except for weight segregating
by weight)

So, will another $140 for a reamer and $600 for a new barrel get me another 0.3 in accuracy?

 

[fdshuster]

Maybe!

 

[NorCalMikie]

And maybe not. I've got some Sellier & Bellot 55 grain ammo that will produce 1/2" groups without doing any extra work.
And that's from 100 yards off the bench. I keep it stashed for when I "need" to hit something. 8)
I think Frank just about covered everything. Keep playing till you find the right load.

 

[Rock Knocker]

My AR wouldnt hit a thing with 69gr right at the lands. I even cut the front of my mag off to have it cycle longer rounds. Loaded right at 2.250 (.023 jump) im getting -.5 MOA. I shoot with a couple serious AR 10/15 shooters and they all jump at least .010" weather 308 or 223.

 

[dan06]

Cut out the front of the mag first and seat them longer.....I cut out 20rounders so they will work with 10 rounds and gain quite a bit. I know that my rifle likes 60gr sierra vt bullets seated as far out as possible with a cut mag. I'm getting 5 shot groups consistantly under .5" This makes it a nice coyote rifle with pleanty of back up rounds incase they bring friends in for dinner at the bait pile.