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Old December 23, 2011, 12:28 PM   #7
Bartholomew Roberts
Senior Member
Join Date: June 12, 2000
Location: Texas and Oklahoma area
Posts: 6,776
Gas piston or DI? I'd read that the piston system was cleaner, cooler, and more reliable. The last bit especially in a short barrel system. OTOH, builders like Noveske make some pretty short setups using the DI system. What's the for Dummies explanation of the differences in the two, especially as it relates to an SBR that may or may not have a can on it at any given time.
The M16 was originally designed with the idea you would have a 20" rifle with a gas port at about 14" or so down the barrel. This means that the port pressure would be approximately 12k-15k PSI and that the dwell time would be whatever time it took for the bullet to travel the remaining 6" or so and exit the barrel.

When the barrel is shortened in a direct impingement system, you have a couple of different issues: 1) You still want dwell time to be close to the original; because you are using the same M16 parts in the receiver. 2) You are tapping pressure further back in the barrel, meaning the port pressure is higher.

Normally, what you would do is enlarge the gas port (or reduce it) so that you tap off enough gas to cycle the action with whatever dwell time you have. However, when you add a suppressor to the mix, you are adding a device that adds an extra 6" or so of dwell time and is purposely designed to slow the exit of gas from the rifle - which means that the same gas port that works with a 10.5" unsuppressed barrel is now overgassed when the suppressor is attached. Likewise, if you design it for use with the suppressor, it will be undergassed when the suppressor is not in use.

A gas piston is a little less susceptible to these issues, although it will still be affected by them. It won't necessarily be more reliable though and since a suppressor will blow a lot of gunk back into the action regardless, it isn't going to be dramatically cleaner than a suppressed DI.

AR, M4....What gives? What exactly are the differences here and does it even matter given what I'm looking for?
The differences aren't really relevant for your purposes. AR or Ar15 can refer to any of the many dozens of variants of the M16 family of weapons. An M4 is a specific variant with specific features - it is what is currently the general issue rifle for the Army, although the term will also be used to describe M4 look-alikes that are semi-auto (also called M-4geries).

've also noticed platforms advertised as having a full auto bolt/carrier what not and wondered what difference this made since new manufacture F/As are not allowed for the general public. Is it just a durability issue, and if so what other "full auto" pieces/parts should I be looking for in a build?
The differences between a full-auto bolt carrier and a shrouded semi-auto bolt carrier are mainly in how long it holds the hammer down as the bolt carrier cycles. As long as the firing pin on the bolt carrier is shrouded, both should be equally reliable in a semi-auto build, though the full-auto will be a touch heavier which may delay unlocking a few microseconds more.

This kind of leads me to an unrelated question that is strictly a curiousity thing. What exactly is the difference between a semi auto AR and a full auto version? I've never looked it up specifically but bits and pieces I've read here and there made it seem the difference were slightly more than minor which confused me. Explanation below.
Usually the following parts: bolt carrier, firing pin, disconnector, safety, hammer, trigger and sear.

Given your requirements, you might take a look at a 9" AR upper in .300 Blackout (google "AAC Honey Badger"). I haven't ever played with one and marketing hype is just that; but according to the marketing hype, the .300 Blackout was designed for the exact role you are looking at. Only issue is that cost of ammo would be significantly higher than 5.56mm.

Assuming the marketing hype is true (big assumption), I would recommend it over 5.56mm for the following reasons:

1. A 7-10" barrel on a 5.56mm AR is difficult to get running well for all but the most experienced makers. The same length barrel AND occasional suppressor usage is going to be a challenge.

2. The 5.56mm relies on velocity to be effective. 10" and less barrel means you are sacrificing most of what makes 5.56 so useful. You lose velocity but you keep the same small projectile.

By contrast, the .300 Blackout was designed from the get-go to be fired from short (12" and less) barrels in direct impingement ARs with both suppressed and unsuppressed use. In addition, under 200m, it is going to offer more energy because it will have a bigger bullet and it is designed to make the most out of shorter barrels. You'll also have the option of using 220gr subsonic rounds that will make the rifle as quiet as an MP5SD (at the expense of terminal performance and a rainbow like trajectory even at 200yds).

Again, I haven't actually verified any of those claims or even taken time to consider whether they could possibly be true. I'm relying totally on marketing hype unfiltered by any skepticism at all.
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