AR Terms - Define for a newbie

M

MAArcher

New member
What does this stuff mean:

"UPPER RECEIVER: Mil-Spec with Indexing Marks and M4 Feed Ramps." - What are indexing marks and M4 feed ramps?

"Mid-length Gas System" and "Pinned Low Profile Gas Block CNC Machined of 4140 hardened steel and Mil-Spec Heavy Phosphate Coating, Mid-length, Direct Impingement" - what does all that mean? How does it compare to other options?


"BOLT CARRIER GROUP: M16 Profile, Mil-Spec MP Tested, Chrome Lined, Properly Staked Gas Key" - What other profiles are there and is the M16 profile better? Whats a propertly staked gas key?

Thanks
 
I believe the indexing marks are referring to the numbers on the picatinny rail that you can use for reference. Some receivers have these numbers while some don't.
example:
b063fae6ed6597126ff5cc999bf11158_3.gif



M4 feed ramps are the ramps machined into the upper that line up with the feed ramps on the barrel extension.
As seen here:
eZRZLWs.jpg


Mid length gas system is a gas system that is between carbine length and rifle length. It should be a bit softer on the gas system than a carbine (shorter) system. Basically the gas block is further down the barrel than a carbine length.

The pinned low profile block just means that it's a small gas block that will fit under the handguard. It's pinned to stay in place so it doesn't move.
Here's an example of a low profile block:
Wilson-Combat-Adjustable-Low-Profile-Gas-Block-1.jpg
 
Here are some of the different BCG profiles:
ar15carriers.jpg


M16 is most common and usually sought after.

The properly staked gas key is to make sure the screws don't back out while using the rifle.

example:
ar-key-stake-2-082712.jpg
 
Well, here's a thank you from another member pretty new to AR's!

Edit: some of this I sorta figured out on my own reading different source, some I was scratching my head. Your photos make it easy to grasp the differences, so thanks again.

JPK
 
heres a good comparison video of bolt carrier groups and their pros and cons.

heres a good comparison video of gas system lengths

AR15.com is an awesome source of information as well as entertainment
 
If I recall, the difference for an M16 bolt carrier and an AR15 bolt carrier made by the same manufacturer is generally less than 1/2oz. So comparatively, going from a standard carbine buffer to an H, or an H to an H2, and so forth, would have about twice the recoil reducing and cycle rate retarding effects as changing out the BCG.

There's really nothing that makes the M16 bolt better than an AR-15 bolt - the under belly of the tail is the only difference, which is what engages the auto-disconnector for select fire weapons.
 
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