.223 vs 5.56 Chambers - HUGE bullet jump?

[bookworm]

5.56mm NATO versus .223 Remington Chamber Differences - Page 1 - AR15.COM

From this link above...if my math is correct, the freebore (distance bullet travels before engaging lands) difference between .223 and 5.56mm is 0.152" (0.040 + 0.045 versus 0.073 + 0.164) Is my math correct? When I look for other information online I get most people stating a MUCH smaller difference in freebore between these two chamberings.

I have a bushmaster in 223/5.56 chambering and am finding a huge bullet jump when I load my COL to 2.25". My bullet jump is around 0.174" and that seems HUGE to me...so I'm trying to figure out if that is normal or not.

This rifle has never shot very well with factory ammo and I'm trying to work up some loads that might improve accuracy. This is the first possible "strange" thing I've come across that might be a clue to my poor accuracy.

I know that this question is not a "long range" question per se, but I'm trying to turn my AR into a decent varmint rifle...so I'm hoping one of you experts out there might be able to help.

Thanks for any insight on this.

 

[Kevin Thomas]

Bookworm,

To begin with, what ammo have you shot in this rifle, and what kinds of groups have you seen? If you've been using the standard M193 55 grain FMJ stuff, three MOA is not at all uncommon for that. Not accurate by most of our standards, but perfectly adequate for what the rifle/ammo was intended for. If you want to use better ammo in there, I'm sure the rifle will deliver far (FAR) better groups than this with little difficulty.

Since you're using this as a varmint gun, I assume you'll be using the 52-53-55 grain bullet range? These will shoot just fine, even in a fairly fast twist. I also think you're worrying far too much about the jump. Your OAL is basically set by your mag length, period. Unless you take some drastic measures such as mag alterations or single loading, you're going to be at 2.260" (max), end of story. Don't sweat it, they shoot fine at this length.

If you're interested in accurizing the rifle, the first and most important step you can take is to float the barrel. There's any number of float tubes out there, and all of them will improve the inherent accuracy of the rifle. Next step after that is a better trigger, and again, there's plenty of choices out there. With these two alterations, it's rare to find a properly built AR that won't shoot well under MOA, usually with a fairly wide variety of components.

So, what are you using for ammo now, and what types of groups have you seen so far?

 

[Trickymissfit]

5.56 and a .223 are the samething. The difference between the two rounds are the primer crimp and bullet crimp. But on the otherhand the throats are cut much differently. A .223 labled round shot in a 5.56 chamber will show a little more chamber pressure, but nothing to be alarmed about. A good read on the differences between the two can be found in one of the back issues of Rifle Magazine. This article really goes into depth with very nice chamber prints (as well as throat drawings too)
gary

 

[Loner]

The 5.56 is throated longer to accomodate higher pressures it is loaded too. In theory
(and in actuality) a 5.56 can stretch the case enough to pinch the bullet in the case by
expanding into the throat of a .223 chamber.
There are several .223 reamers and several 5.56 reamers. Not only is the neck dia.
larger on some the freebore dia is larger, the throat angle less and the freebore longer
and throat becomes longer due to it's wider starting point and lesser angle to get back
down to bore dia.
In a nut shell, if you have a 5.56, you reload, and you have a fast twist like a 1:8 or
1:9 go with longer bullets in the 70 grain range and you can load out to the lands.

 

[bookworm]

Hi fellas - thanks for the replies.

Most of the ammo I've put through it has been some PMC 55gr FMJ BT. The barrel is 1:9. This ammo has produced roughly 2.5 moa. I know others with pretty standard AR-15 setups that are getting moa with this ammo...so I was hoping for better.

I'm in process of loading some 69gr SMK's with Varget as propellant. My hope is to get something closer to moa performance with these rounds. It was in the loading process where I discovered this very long freebore/jump and am wondering if this is normal or not. I know it certainly is not for bolt rifles. I've read a few other sources (not necessarily reliable) that say the difference in freebore between these two chamberings is an order of magnitude (10x) smaller than what I'm finding.

The barrel is floated, but you make a good point on the trigger - it is terrible. It is on my list to improve...as is fire lapping.

Anyway...I'm still hoping to get some solid confirmation that my chamber freebore is normal so I can cross that item off my worry list.

Thanks for the discussion.

 

[bookworm]

Loner - saw your reply after my last post.

In theory I could load almost to the lands with the 69gr smk's, but there would be barely any neck holding the bullet in...and the larger problem is the magazine size. I would like to use the magazine, which has a limit of 2.26". I knew at the outset that I would have a larger jump due to the 5.56 chamber and the magazine size. What I was not mentally prepared for is a jump of 174 thou !! I'm used to talking in single digit thou when discussing bullet jump.

 

[Kevin Thomas]

Bookworm,

The accuracy you're getting out of that FMJ stuff is probably about all that you're going to get out of this type of ammo. As I said, 3 MOA is perfectly acceptable for this, and you'd likely see the same groups out of an accurate bolt gun. Bottom line, you can't make crap ammo shoot like NM stuff, period.

A good match-grade bullet will make a tremendous difference here. Varget and RL-15 are the two go-to powders for competitive Service Rifle shooters. Both are good, choice is all yours. If the rifle's already set up with a float tube, it should be capable of MOA or better as is. The trigger's always an issue, but as I said, there's lots of options out there.

Again, forget about the jump, it's not a problem. I'd also strongly suggest you forget about 90% of what you've learned from loading bolt guns; gas guns are differernt, and NEED to be treated differently. If you're really (and I mean REALLY) hung up about the jump, the forget the magazine and accept the fact that you're got yourself a semi-automatic single shot. The LR VLD style bullets are intended to be loaded well beyond mag length, because the 600 and 1000 yard stages of High Power competition require the rifles to be single-loaded. They were never intended to be used from a magazine, and that's how they were designed. Even there, the painstaking approach to seating the bullets .xxx" off the lands is wasted on a Service Rifle. As soon as you ctrip the bolt, you've just soft-seated that bullet, I guarantee you.

 

[bookworm]

Thanks for the advice Kevin. I want to load in the magazine, so I'll forget about the jump and press on. I wasn't so much hung up on the jump as I was concerned that there was something wrong with my chamber. I'll assume everything is fine as is and move on.

Just loaded some rounds with Sierra Match King's and Varget...ensuring bullet runout is 0.003" and less.

I'll give these a go in the next couple days and see what I get. If it still won't shoot, then I'll be back with more questions

Thanks again.

 

[Kevin Thomas]

Bookworm,

If you're fairly new to the gas guns, it might be worthwhile for you to take a look at Glen Zediker's book, "Handloading for Competition." Glen deals with Service Rifles at length, and it really is a very different world than loading for bolt guns. Some things which we take as critical for "normal" reloading become completely irrelevant, and in some cases, downright problematical. Others, which you have some latitude with in bolt gun reloading, suddenly become critical (such as full length sizing, shoulder bumping, etc.). Bottom line is, they're different, and can't be approached the same way.

The AR is an inherently accurate little rifle, and with a few refinements (such as the float tube and a decent trigger), they're simply phenomenal. They've completely displaced the larger 30 caliber Service Rifles in competition these days, and they serve as the basis for some very accurate varmint guns. A friend of mine, John Feamster, actually shot one in a registered BenchRest match some time back. This wasn't well received by the other competitors, who tried to throw up some roadblocks to keep him out of the match. Nothing in the rulebook prevented it, ans when he cheerfully agreed to use a brass catcher to prevent his ejected cases from annoying the other shoters, they grudgingly let him pay his entry fee and shoot. They were even less amused when during one of the first relays, he fired an "Official Screamer" group; a group fired in a registered match measuring less than .100" at 100 yards, .200 at 200 yards, etc., for which an award is made. He didn't win the match, but he did beat several bolt gunners, and placed somewhere in the middle of the pack. Not too bad, considering the company. They also made it plain that he wasn't really welcome at future matches. Hey, some folks just can't deal with progress.

 

[whitely]

Have a look at this page (223 Rem + 223 AI Cartridge Guide). There is also a link to the sierra loading guide for the AR. I have a 20" RRA upper, and with the 69 gr match kings, I can shoot 1/2" groups without too much trouble. I have an 8" twist, so the 60-70 gr bullets work fine. I have also found some good loads with the 53 gr bullets, both boattail and flat base.

One of the things I love about shooting the AR at varmints is that you can load a 20 round magazine up, and get in a good position for many shots without having to manipulate anything (bolts). Makes for quick follow up shots on those crowded mounds.

 

[bookworm]

Kevin - I'm about 2/3 of the way through Zediker's book right now...very informative and entertaining read.

I went out and shot today and got mixed results with various loads. I varied the charge from 24.5 to 26.0 grains of Varget (at half grain increments), and shot two separate sets of 3-round groups for each load.

Overall I was a bit disapointed with most of the groups coming in at around 1.25 - 1.5" at 100 yds. The one exception was the 25.5 grain loads, which both grouped at just over 0.5"...so it gave me some hope to build on.

One question I have for you: the loads above (26.0) and below (25.0) opened up significantly - to around 1.5". Since 0.5 grains is quite a bit this was not a huge surprise. I also loaded 3 rounds at 25.3 grains and I was surprised to see those throw a group also near the 1.5" range. Is it possible that 0.2 grain difference (25.5 to 25.3) could make a group open up from 0.5 to 1.5 inches? Seems to me it should not be that sensitive.

Other possibility is that my two 25.5 groups had some luck involved. Interested in your thoughts.

visigoth - thanks for the link. The 6mmbr site is a good one - I'm always amazed at the amount of good info on that site.

 

[Loner]

25.3 is Sierras best accuracy load with the 69smk. No surprise you fall right in that
range.

 

[bookworm]

Loner - what is surprising to me is that my two best groups were with 25.5 (at 0.5") and that 25.3 opened up to 1.5".

I would have thought that 0.2 grains would not have shown such a large difference.

 

[Loner]

It's a small case so .2 is a large jump percentage wise. If it were a mag round it would be like a .6 jump.