Beyond “The Chart”

Most AR-15 enthusiasts, especially those who spend time online, are familiar with “the chart” – a document comparing M4 type carbines from various manufacturers via a number of features. These features are generally checked or unchecked, and weapons that check more boxes on “the chart” enjoy a higher reputation than those that do not. For the most part, I agree with that assessment. I would certainly rather have a “chart checker” than a weapon related to an M4 carbine in appearance only, especially considering the rapidly closing price gap between the “cheap” and the “expensive” rifles.

However, there are a number of other factors that have an effect on the reliability or “shootability” of an AR-15. I will attempt to cover a few of them today, with the possibility of more down the road. Many of the items that I compiled are things that a lot of people take for granted, or don’t think about. I’d like for people to at least think about them, even if they decide to ignore them after reading this article.

Gas Port Diameter

Gas port diameter and location is crucial to how an AR-15 operates. These items are selected based upon a number of factors, which include but are not limited to:

  • Barrel length
  • Intended ammunition
  • Intended reciprocating mass
  • Intended use
  • Environmental conditions

In other words, the AR-15 should operate as a system, not as simply an amalgamation of parts that fit together. A quality rifle will be tested and assembled with all of the above factors – and more – in mind. Some manufacturers simply guess at what conditions the rifle will see – or don’t give it any consideration at all – and put out rifles with larger than standard gas ports. As stated, this varies, but the gas port on a 14.5″ Colt M4 Carbine will generally be ~.062″. To function with a 10.3″ barrel, the armorers at NSWC Crane enlarged the gas port to .070″. Commercial manufacturers regularly put out 16″ carbine barrels with .09″ gas ports.

Gas port roughly .090″ in diameter – this is not the ideal method to determine exact gas port diameter, but it will get us close.

Such a large gas port is unnecessary and detrimental to the reliable function of the weapon and degrades its recoil characteristics. Now, before anyone says “BOY, TRY MAH .338 WIN MAG BEFORE YOU TALK ‘BOUT RECOIL!”, remember that the original purpose of the AR-15 was to put rounds on target in a combat situation. To this end, being able to get back on target quickly is very important. It’s much easier to rapidly engage a number of fleeting targets at 300 or 400 yards with a weapon that stays on target than it is with a weapon that comes off the target.

Smaller .068″ gas port on Spike’s Tactical carbine. All else being equal, this weapon is softer shooting than the above carbine with the .090″ gas port.

Regarding function, the carbine length gas system is not the optimal choice for a 16″ barrel due to dwell time – the time between when the bullet passes the gas port and when it exits the muzzle. In other words, during that time, gas is coming back through the gas tube and acting on the bolt carrier group. An “overgassed” weapon can result in an abnormally high reciprocating component velocity, which can lead to malfunctions such as “bolt over base” (to put it simply, the carrier outrunning the magazine spring’s ability to move the top round into place) and increased wear and tear on various components. Carrier velocity will generally increase if a suppressor is used. During full auto fire, this will also lead to bolt bounce, where the bolt literally bounces off the barrel extension, thereby coming out of battery and preventing the weapon from firing.

A weapon that has been “tuned” to shoot a certain type of ammunition – such as the Knight’s Armament SR-15E3 or Bravo Company 14.5″ Midlength, among others – has outstanding recoil/muzzle jump characteristics, but maintains complete reliability with almost all types of ammunition. Proper function should be established with “serious” ammunition before the weapon reaches the hands of a consumer. In the case of a home built AR-15, consideration should be given to the above factors, and any additional ones the builder wishes to include, before buying a barrel with a large gas port.

HP/MPI Test Variations

Ah, MPI (also known as MP or magnafluxing, and short for Magnetic Particle Inspection) testing. A soothing topic that everyone agrees on.

Actually, I’m lying. I’ve heard everything from “MP testing causes problems” to “Anything that is not MP tested is junk.”

The truth is somewhere in the middle. First off, MP testing is considered NDT, or nondestructive testing, and will not in and of itself cause problems. Second, there are a lot of parts that are not MP tested and which I would not consider junk. I might not choose them over a comparably priced, MP tested part, but I would not call them junk.

What is the purpose of this testing, and what parts are tested? Well, generally, the bolt and the barrel are tested. These items are generally tested with a proof load, or a single overpressure round, before they are MP tested, in order to highlight imperfections. Generally, the purpose is to find imperfections, be they cracks, voids, inclusions, or stringers. Generally…you get where I’m going with this?

Bravo Company MFG HP/MP tested bolt

There are variations in the testing, and there are variations in standards/rejection criteria. “The Chart” covers batch testing versus individual testing, but not much else. A company could theoretically say that they performed individual MP testing of bolts and barrels, but if they didn’t reject any items, the testing would be worthless. Similarly, they could throw out parts that had obvious defects such as cracks, but not parts that had other defects as mentioned above. It’d be like Nevada requiring STD tests for all the registered prostitutes, but only pulling the ones that had AIDS.

Furthermore, barrel MP testing is closely related to barrel steel – that is, MIL-11595E certification. According to Bravo Company MFG, barrels manufactured of non-certified steel can have failure rates – as per ASTM E1444 standards for MPI testing – of 30-40%. That’s unacceptably high for any profit-driven company. Therefore, many companies don’t bother with MP testing, because a third of their products would fail the test. In addition, the certified steel is only available in large quantities, necessitating a large outlay of funds, which prevents many companies from utilizing it.

Bolt failure at the cam pin hole is a more common failure with non-MPI tested bolts, because these cracks are easily detectable by proper MPI testing. This is not to say that all non-MPI tested bolts will fail – there are other factors, such as heat treatment and steel utilized – only that some companies that bypass MPI testing also bypass the quality standards required to produce good bolts in the first place.

So the next time you hear a company say that they MP test their products, ask them what their acceptance/rejection criteria is. You might be surprised at the answer.

The Action Spring

Of all the springs critical to the operation of an AR-15, the action spring (that’s “the big one that goes in the stock”, for those that don’t know) hardly ever gets any love. Apparently, most people assume that there are only two types of action springs – carbine and rifle – and that they never need to be replaced. Nothing could be further from the truth. Just as Glock and Kimber recommend regular replacement of their recoil springs/recoil spring assemblies, it is a good idea to periodically replace the action spring. Now, plenty of people will say that they have fired ten thousand rounds through their AR-15 without ever changing the action spring, and they didn’t have any problems. They are being honest.

However, in a rifle with a very high round count that has started to malfunction, replacing the action spring – as well as other springs such as the extractor spring – can breathe new life into an old rifle. Even new rifles can benefit from a quality action spring, as the consistent fore and aft movement of the reciprocating assembly is an important factor in maintaining complete reliability. Some cheaper stock kits come with action springs that are of unknown origin and rate. Given the importance of the action spring and the extractor spring to the proper operation of an AR-15, I would not go cheap on either one.

Companies such as Sprinco offer top quality action springs designed for specific applications. If you have $22 (plus shipping) to spare and have springs of unknown origin in your rifle, you would be doing yourself a favor by ordering their action and extractor spring kit. Even if you have quality parts in your rifle, it’s always good to have spares.

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