80 grain SMKs in a Krieger min spec 223 barrel?

[DarthNul]

I'm building an AR-15 for high-power service rifle competition. I purchased a Krieger 1 in 7.7 DCM HBAR barrel from Brownell's for the project. The barrel has a "min spec" 223 Remington chamber. The description on Brownell's web site mentions that this barrel has the "ideal" twist rate for 69 to 80 grain bullets.

I took my Hornady OAL gauge to it tonight with 69 gr SMKs, and both 69 and 77 grain Nosler CC bullets. I was a bit surprised to find that at maximum magazine length (2.260) OAL, both of the Noslers were touching the lands, and the 69 gr SMKs touched at 2.270.

I don't have any 80 gr SMKs in the house but I was planning to use 80s at 600 yards. Knowing that 80 gr SMKs cannot be safely loaded to magazine length, I'm wondering if I should return this barrel.

Anybody out there loading 80 gr SMKs with a Krieger "min spec 223" chamber? What OAL do you load to?

[Bart B.]

200 yard standing and 600 yard prone matches in high power service rifle matches are shot slow fire; single round loading with one round charged in the top of the magazine or inserted directly into the chamber. Rapid fire matches are shot with loaded magazines.

One exception is shooting the Nat'l Infantry Trophy Match when you shoot rapid fire (24 rounds in 50 seconds at 600 and 500 from prone, 300 yards sitting and 200 yard standing if you have any ammo left over. You'll reload once or twice in each stage. You use lighter bullets for this "rattle battle" match so they'll fit in the magazine.

Check the rule books for details:

http://thecmp.org/wp-content/uploads/Rulebook.pdf

http://compete.nra.org/documents/pdf...R/hpr-book.pdf

'Tis good to see folks starting out with the AR15 for rifle competition. I'm the first person on earth to complete the standard course of fire with the 5.56 NATO round shooting a custom modified M16 at the 1971 Nationals; DCM Service Rifle Competition. It was a lot of fun.

[Jimro]

A 223 Min Spec barrel means you will have the shortest throat dimensions of any commercially available chamber.

You will see pressure signs at lower charges than someone with a Wylde chamber or 5.56 chamber, simply because your bullets are impacting the rifling sooner. But, if you do your load workup you will be safe, and you can have a very accurate rifle. As the throat wears you can lengthen your bullet seating to "chase the lands" if you like.

Without actually running some loads over a chronograph, I can't tell you if your throat will hurt your 600 yard scores or not. As long as you can make 2,550 fps or faster with an accurate load you won't be able to blame any bad scores on the rifle/load.

But when I bought my Krieger for my service rifle, I chose a Wylde chamber.

Jimro

[SonOfGun]

I use 69 SMK loaded at magazine length for 200 and 300 stages.

Then use 80 SMK loaded long for chamber length for the 600 yard stage

shot single shot slow fire.

[rg1]

Just for comparison: My White Oak Armament built NM upper with a Krieger barrel with Wylde chamber to touch the lands with a Sie. 80 gr. MK the loaded round overall length is 2.472" and WOA recommends a .005"-.015" jump to rifling. 77 gr. MK's in my rifle get loaded to magazine length but just to check the overall length for 77 MK's to touch the lands in the Krieger with Wylde chamber of my rifle is 2.316". Hornady 75 A-Max's which have to be loaded into the chamber one at a time, the oal to touch the lands is 2.430". Not an experienced long range match shooter but if I were building a National Match rifle I'd prefer the Wylde chamber for sure. These dimensions are just for comparison and in my WOA built NM upper and other rifles may differ. These measurements are overall length and not measured with a gauge that measures to the ogive length to touch the lands.

[Unclenick]

Darthnull,

It's not a problem. The 80 grain bullet at 2.550" COL has such a long nose that its ogive meets the lands at about the same place as the shorter nose of a 69 grain SMK seated to 2.260" COL does. As my illustration below shows, nose shape difference can give you a very different COL even when the lands are the same distance from the bullet ogive.



Even if the above were not true, HSM produces 2.260" COL ammunition loaded with the 80 grain SMK. They just seat it deeply enough so there is a very small gap between the case mouth and bullet ogive. It's less than you'd expect because the SMK is a tangent ogive bullet, which means the initial departure of the ogive radius from the bearing surface is very gradual, and the brass springs back a little after the bearing surface passes below the mouth. So this arrangement will shoot fine. You just can't use the same load data that is used with the bullet seated to 2.550". In QuickLOAD I find I am reducing charges about 8% for that shorter seating. That will cost you a little velocity, which you probably don't want to give up for 600 yard shooting, as it means a little more wind deflection.

[rg1]

One good article about AR chambers for long bullets:
http://www.google.com/url?sa=t&rct=j...ki92giehBqDCMg
Lot's of info by searching AR 15 chambers.

[DarthNul]

Thanks for all the replies!

I hope to pick up a box of 80 gr SMKs at my local reloading shop over lunch time tomorrow. I'll see where they hit the lands tomorrow night. 69 gr SMKs are on the lands at 2.263 OAL with this barrel so things are looking better already. It's good to see the recommendation to put the 80 SMKs only .005 to .015 off the lands. I was afraid I might have to make some serious sacrifices to go much less than .030 off.

So far I've measured 69 gr SMKs, 69 and 77 gr Nosler custom competition HPBT, 75 gr Hornady HPBT and 55 gr Hornady bulk FMJs. One things for sure: I won't be shooting any of the stuff I've loaded for my 5.56 DPMS 18" HBAR in the new gun. Other than plinking ammo with the bulk Hornady 55s, it's all at 2.260 OAL, and at least .020 off the lands.

I won't be doing a lot of 600 yard shooting but since this will be my "no excuses" gun, I do want to have the capability when I get the chance.

[DarthNul]

I pick up some SMK 80s at lunch. They touch the lands of the new barrel (without sticking) at 2.415. So it looks like my min OAL will be 2.4 and max will be 2.41.

If I go with a 2.405 OAL I'm halfway between 2.260 magazine length and the 2.550 OAL recc'd by Sierra. I wonder what the charge reduction would be for that, and what kind of velocities to expect...