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.
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?
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)
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.
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.
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.
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.