I'm really starting to develop and interest in this chambering in an AR. What I have read so far is that a quality barrel and bolt are essential, and that fits well into my beliefs on any firearm. Stuff I'm not real sure of are as follows;
Stocks and buffers. For this rifle I would prefer to use a fixed stock. A couple I have in mind are an old A1 stock if I can source another or the Magpul fixed stock. Can the standard rifle buffer be used and still be reliable.
Barrels, handguards and sights. If I'm reading right it seems the gas block to be used is .875". Am I limited to the standard under the free float types or is there a fixed or railed offering available? I actually prefer fixed, but I'm not a snob in that regard and have some free floats as well. I just don't really put much beyond optics are my rifles in most cases. Are 16" barrels the longest available.
Uppers and lowers. I have already found the information about these parts and will be ordering an Anderson stripped upper with the opened port, and I got a couple of free lowers already laying around.
Magazines. I will be ordering some magazines and probably end up using them only for this rifle, so they might get modified if needed for optimum operation. I got plenty of 5.56 mags as is so this option won't bother me at all.
Range. Just how far will this big bore reach out there? I am used to shooting .45-70 at 500 meters, but that is with a Vernier sight.

Thank you in advance for any guidance.

TL;DR.

No issues at indoor range (80 feet max).  When it warms up we will try it outside.


Just one word:

TROMIX

Pmags are generally a no-go. The front of the magazine has a rib that keeps the follower from tipping up and straddles the staggered 5.56 rounds. You can dremel out the top of that rib but the follower will tip up when unloaded and the bolt will drag on it if you let the bcg forward on an empty mag. Regular aluminum mags give the rounds a little more leg room. I polish the underside of the feed lips so they don’t scratch up the brass. Some of us dremel out a bit of the front edge of the mag into a U shape so rounds can be manually pushed out of the magazine, but in operation you shouldn’t see any feed issues.
As far as barrel and gas block concerns, I would consult with the mfg of the barrel you want to purchase. My .458 gas gun has a .936 bull barrel and I’m not sure a free float that covered the gas block would fit unless it was a really large handguard with respect to the inside diameter.

The .458 is designed to work with a regular carbine buffer system, don’t overthink it.

Regular CAR buffer and spring OR rifle buffer and spring.

Any size gas block will work, it doesn’t matter, .750, .875, .936, you choice depending on the barrel profile you want.  I don’t think a pencil barrel with .625 block is a good idea.

Gas length is another thing.  14-18” you want CAR length gas.  12” and under you want pistol length gas.  You DON’T want midlength gas and rifle gas is straight out, ALWAYS.

Twist?  1:10 to 1:18 work fine.  Super loads any will do any bullet weight, but if you want heavy/long subs a 1:10 might be a little better.  I have all and there’s not a lick of difference with supersonic loads, and very little with subs up to 500 gr. But 1:10 may be better with 500 gr. plus bullets or long copper bullets

 With midlength gas you cannot use powders that give you the highest velocities and by going to slower powders you need with midlength you end up with the same velocities you get in a shorter barrel with CAR gas.  Huh?  My 20” with midlength gas gives the same velocities with slower powders as does my 16” with CAR gas with faster powders.  But longer 20” barrels with faster powders does not increase velocity over a 16 inch.  So why bother?

Stocks, your choice.  The A2 with standard butt plate is a meat grinder unless you put on a KicK-Eez recoil pad.  Most CAR type are OK too but I’d avoid standard CAR and M4 stocks, they have been known to get damaged or collapse.  My favorites are the MagPul MOE rifle stock or the CTR with Limbsaver pad but most other quality stocks work fine also.

The accepted range is 200 yards or less.  If your particular bullet won’t expand past a certain velocity then the range it drops below that velocity is your maximum range for a clean kill, unless using wide meplat bullets, but their poor B.C. makes the bullets really start to drop past about 150-175 yards.

Anderson enlarged port uppers are fine.  I have several and no problems.  Go with a TROMIX barrel and bolt and you’ll have no trouble.

16” is the longest offered because as I alluded to above, you don’t gain anything really when you go longer.  My 28” non gassed gives me essentially the same velocity as my 20”, 18”, and 16” barrels with the same loads so whats the use of a longer tube?

Standard 5.56 mags, GI aluminum or Lancer AWM, need NO modifications to work, no fancy follower, no tweaked feed lips, NOTHING.  The half moon cut in the front is only needed with really wide meplat bullets OR to make unloading unfired rounds easier.  The cut does NOT aid feeding with spitzer, RN or most standard HP bullets, only cast wide meplat bullets.
PMags, as mentioned, are an often no-go because of the front center rib.  If you want to seat a little longer than 2.250” then PRI 6.8 mags work equally well with NO mods allowing you to go to 2.300”.