Most Aero Precision EPC 9mm parts seem like a good value for 9mm AR builders. However, I would avoid 3 of the parts, substituting with different options.
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Synopsis:
The three Aero EPC parts I would not use in my build are the EPC bolt, the EPC buffer, and the EPC buffer spring.
The Bolt:
The EPC bolt, as of late 2022, has undergone a design change and is missing the bottom feed lug from the bolt face.
As I show in this video, the bottom feed lug helps prevent out-of-battery discharges as the cartridge is pushed out of the magazine and to the chamber. Without the feed lug, the cartridge could potentially be fired out-of-battery if the bolt stops short before it fully chambers the cartridge and the trigger is pulled. The stoppage could be caused by just a dirty chamber, dirty action, debris stuck in the chamber or action, the magazine rubbing on the bolt, etc. I had this happen to me with a different bolt (not EPC) that also has no bottom feed lug. The magazine feed lips were dragging on part of the underside of the bolt, stopping it before the round was fully chambered. I pulled the trigger and “kaboom”. Brass shrapnel everywhere and powder burns on my hand and face.
I’m confident this OOB detonation would not have occurred if the bolt had a bottom feed lug.
The EPC bolt is also a 2-piece, or split design. This means the back portion of the bolt is a separate piece pinned onto the front section.
As far as I can find, this provides no significant advantage over a normal 9mm AR bolt design. I suspect it’s just cheaper to make. As a result of the split design, the EPC bolt can be more difficult, if not impossible, to use with folding buffer tube systems or additional bolt weight options.
I also received a report from an owner that after several thousand rounds their EPC bolt cracked at the point indicated by the red arrow in the photo below:
The crack occurred where the body material was the thinnest. That pin, and the material around it comes under a lot of stress as the bolt slams forward and back, holding the heavy rear weight in place. It seems that this bolt design may have inadequate material around the retaining pin.
Bolt Alternatives: (FYI, I do not receive revenue from ANY of the links in this article.)
Any 1-piece bolt that uses a 5.56 external extractor and has a bottom feed lug…
- B. King’s Firearms 9mm bolt $ (nitride, Best value – IMHO)
- Kaw Valley Precision 9mm bolt $$ (nitride)
- Faxon 9mm bolt $$$ (nitride)
- Angstadt Arms 9mm bolt $$$ (nitride)
- Brownell’s 9mm hybrid Colt/Glock bolt $$$ (NOT “Glock-only”!) (nitride)
- FailZero 9mm BCG $$$ (nitride/EXO)
The Buffer:
While the Aero EPC buffer is a good weight (7.7oz.) and proper length for 9mm (about 4″), it is a “solid buffer”. It’s a solid hunk of steel with no internal sliding weights, which makes is susceptible to bolt bounce. Being solid, it’s the cheapest design to manufacture.
When a bolt “bounces”, the bolt chambers a round, and then partially extracts the round as the bolt bounces backward, a bit like those desk toys with the ball bearings on strings.
If the hammer comes forward as the bolt is bouncing back, the cartridge can fire out-of-battery and blow high pressure hot gasses and brass shrapnel everywhere. This can damage the gun and cause injury. It doesn’t happen frequently, but it’s best to minimize the chances of it happening at all.
Internal sliding weights inside the buffer help prevent bolt bounce by acting like a deadblow hammer, counteracting the bounce. This is the reason Colt used a sliding-weight buffer for their 9mm AR SMG. Sliding weights make the slight “rattle” you may hear when you shake some buffers end-to-end.
Buffer Alternatives:
7.5 oz. Macon Armory
8.4 oz. KAK Industries
11.0 oz. AR Stoner (MidwayUSA)
Additional info and more options:
What 9mm buffer do I need??? (and spring)
The Spring (recoil/buffer spring):
The EPC buffer spring is a very strong .308 carbine spring. .308 springs were intended for use in beefier AR10 receivers firing the .308 rifle cartridge. .308 springs have been implicated in a number of problems in the 9mm pistol caliber AR platform. They really are not necessary, nor do they provide significant benefits for a 9mm AR. Other than covering up feed issues, the one thing they do fairly well is reduce the annoying AR-15 spring noise during firing. A “sproingggggg” noise coming from the buffer tube is common when using standard carbine springs. The .308 spring is so strong and rigid, it doesn’t reverberate much. Instead of overspringing the system with an unnecessarily strong .308 spring, the noise can be effectively eliminated by using a carbine flat wire spring. However, .308 springs are significantly cheaper for manufacturers to include than flat wire springs (seeing a trend here?).
Recoil Spring Alternatives:
Any standard carbine recoil spring (but will have spring noise)
Flat wire springs (silent, or nearly so):
Wilson Combat Flat Wire
David Tubb Lightweight Flat Wire
David Tubb Standard Flat Wire
Additional info:
Flat Wire Spring Comparison
Carbine spring testing results [Updated 2023]
The Rest of the EPC Parts:
As for the rest of the EPC parts, I would have no problem using them in a 9mm AR build.
They had some feeding issues early on with their barrels, but they improved the “feed cone” at the chamber entrance to allow the barrel to feed much better and added the enhanced feed cone to older barrels when customers reported feeding issues. I have one of their barrels and it both feeds and performs well. It’s very accurate. I prefer Macon Armory barrels, but Aero also seems to be a very good choice. Their EPC barrels are reportedly made by Ballistic Advantage, and BA often has lower prices on their web store. Only the barrels labeled “EPC” have the improved feed cone.
Old vs New:
The EPC receiver set has the last round bolt hold open (LRBHO) on the upper, which is the opposite of most 9mm AR’s, so I would get their EPC upper and lower as a set and only use them together. Some people have still reported feed problems despite using one of the barrels with the new improved feed cone. The Macon Armory EPC mag catch can reportedly fix these problems most of the time.
EXCEPTION: I’d avoid heavy-duty hardened trigger/hammer pins. Stronger hammer/trigger pins were used by Colt way back when 9mm bolts were unramped. These are not necessary today and may result in “egging out” the receiver pin holes. I’d stick with standard hammer/trigger pins.
Additional articles on the EPC:
Aero EPC-9 – what the heck is going on? [Updated: replacing barrels!]
Aero EPC Barrel Feeding Fix Update! (feed cone picture added)
EPC Feed Fix Part 2 – magazine height fix
Tested: Aero EPC Buffer Kit Slow-Mo
OOB warning – missing bolt face bottom lug
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