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Posted: 4/17/2006 6:33:50 PM EST
Ok, this may be a seriously silly question but I have to ask it nonetheless...


Background; was recently playing around with loading some 223 with OAL beyond mag length. Basically, shooting for and overall length that was just .025 inches or so short of the lands. The difference in length between these loads, and the magazine acceptable lengths kind of astonished me.

Question: Why wasn't the magazine in an AR designed to be larger so that the whole cartridge length could be used? Alternatively, why wasn't the chamber dimension reduced to meet magazine length dimensions?

The only answer I can think of is that the reduced magazine dimension increases reliability, specifically in chambering the round, but the gap between maximum OAL (at least in my test rifle) and mag length is fairly significant. I'm curious how the designers came up with the "required" difference in dimensions between the chamber and the magazine.
Link Posted: 4/18/2006 5:26:19 AM EST
What you're measuring is the "throat," or leade. The chamber area forward of the bullet has a section of bore-diameter barrel that has no rifiling. The rifling lands have ramps to ease the bullet into the rifling. The cylindrical section is there to reduce pressure. Specifically, to handle the higher pressure of 5.56 vs. .223, and to deal with longer or harder bullets. The M855 is longer than an M193, and the tracer round is longer still. By having the "jump" the chamber pressure stays where it is supposed to be even with longer bullets, hot or dirty chambers, etc.

Benchrest shooters and long-range target shooters like to load long to minimise the jump. It does improve accuracy (to a small but measurable degree) but it does so at the expense of pressure restrictions. Such loads have to be tailored to the rifle they are used in, and may be unsafe in other rifles.

If the manufacturers were the stretch the upper, lower, magazine and carrier to close the gap you see, barrel makers would simply extend the leade to make sure there was a safety margin.

Load to magazine length, with published data, and leave the fussing to the 600-yard shooters.
Link Posted: 4/18/2006 6:39:20 AM EST
i don't think your question was silly at all and i'm glad you asked b/c i never even thought about that until i read this and thanks pat (my name is pat too) for the info!
Link Posted: 4/18/2006 3:25:38 PM EST
Patrick - thanks for the info, but I'm not sure I agree that barrel makers would automatically extend the leade for the safety margin... you may very well be correct, but it seems to me that dirt and/or fouling of the chamber is probably the answer with the AR.


I shoot non-mag length bullets regularly and practice at 600 and 800 yards. Yes, I have to be very careful when developing loads but the accuracy difference that a heavy bullet, loaded long, can make at those kinds of distances is more than a small diferrence to me.

The thing that grabs me is the difference in overall cartridge length. My slow fire rounds measure 2.490 OAL while a mag-length Winchester white box round is 2.20 OAL. It just seems like the difference goes beyond mere feeding/chambering problems when the chamber is fouled or the weapon has been used extensively.
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