Sizz

I'm hearing some are using .223 (which is suppose to be the 5.56 equivalant) and having miss fires, jams, etc but never anyone that uses straight marked 5.56x45mm Nato.

I've looked on some forums and have read there are some slight differences in pressure , etc.

I've also read the M400 is recommended to shoot 5.56x45mm Nato 62 gr fmj.

The owners manual online shows nothing about .223 Rem only 5.56x45mm NATO so I"m a bit confused as if I should look for straight marked 5.56x45mm Nato or anything .223?!

The barrel is marked 5.56x45mm correct?

I'll be receiving mine in the near future to my FFL, I'm wanting to grab a few boxes of ammo while I can, and would prefer no issues especially at the cost per shot currently.

What's your take on it avid M400 shooters?

Jgdisco2

I don't have a m400 but my barrel is marked 5.56 and I shoot 223 all the time with no issues

Kenny D

You can shoot .223 in a rifle chambered for 5.56 but you should not shoot 5.56 out of a rifle chambered in .223. That being said, the M400 is chambered for 5.56. Being a new in box M400, you can shoot both and both should work fine. Be sure to break down your rifle and clean all the factory preservative from the rifle and relube, as the factory preservative can become sticky and cause failures if fired before cleaning, making the user think that the rifle is defective.
If the rifle is used or custom built, check the stamp marks on the barrel and not the lower receiver, as it could be chambered differently.
The way ammo is getting harder to find, you should still use 5.56 for the first couple of hundred rounds to get it broke in and then .223 should be fine. That is just my recomendation and you should use your own discretion.

Sizz

Thank you!

Will the rifle be pretty easy and understand able to tear down and lube? Any specific portion I need to lube, or any I should not? The manual will be helpful but confusing and over whelming at times.

Thank you again
Sizz

CaptCaveman

Just buy a spray can of BreakFree CLP at walmart in the sports department and remove the bolt carrier group from the rifle and spray the heck out of it and then cover up the chamber with a finger and sprays the guide area where the bolt carrier goes into and then put the wet bolt carrier group back in your rifle and you are good to go. You should shoot her wet for a while till she gets good and broke in..

As for ammo you have a choice of .223REM or 5.56X45 NATO. For just basic shooting, just get what you can get your hands on, but for sniper like accuracy, get some heavy 5.56 or .223 in the 62 grain and up weights. The barrel has a 1:7 Twist rate and therefore is designed to stabilize the flight of a heavier bullet. You should get reasonable accuracy with cheap 55grain ammo with groups in the 1 to 4 inch range at 100+ yards. Have fun with her and keep her clean. The key to the AR15 is to keep everything wet but the chamber and barrel at least until it breaks in then you can focus mainly on shiny bare metal parts indicating points of contact or bearing surfaces.

Sizz

Thanks all,

I'm having a heck of a time finding anything marked soloy 5.56x45mm Nato, everything and I mean pretty much anything is marked .223 Rem.

Thanks again

Doug

I own 2 M440's and a 516. All were srtipped down when new and given the top to bottom cleaning and re-lubed with TB25 grease. Close to 900 roundes of Misc 5.56 and 223 ammo and not one issuse with any of the guns. I do stay away from the cheap ammo though.

Doug

gandalf

My M400 is realitively new less than 500 rounds but handles either .223 or 5.56 equally.

For field stripping and cleaning I would suggest going to YouTube there are some good ones there especially one done by an attractive young lady.
JD

ptco911

I have shot both and not had any issues.

Micropterus

99% of everything I put through my M400 is PMC Bronze .223 55 grain FMJ. Never had a malfunction. It works perfectly.