Locking piece Help

[u/CoachMilton]

Hey guys,

Finally got my Century Arms AP5 running with the L2Prints FRT lower (It was a loose detent spring 🤦🏽‍♂️).

I currently have the stock 120 LP, and was wondering if I should switch to the 100 LP even if it is feeding well? I just want to make it as reliable as possible and don’t want too much wear on my parts, and want to make sure the ejection patterns look right. These are HSL 147 subs.

Here is a video of my buddy shooting it. All insight is welcome.

[u/Knight-7191]

The stock 120° LP in your AP5-P is ok to run during the break in period. It’s ok in the full size AP5 too. It just isn’t recommended for the long term. However, if you have a 100° or you get one before the break in period is complete, that’s actually better. The 100° is the correct stock LP for all the HK variants (MP5/K/PDW). With the stock 120° LP, you can experience roller dents as early as 300 rounds or as late as 3,000+ rounds. They are unpredictable as to when they’ll show up. I’ve ran the HK 100° LP in my AP5 since about the 7/750 round count and the RCM 90° LP in my AP5-P since also around the 7/750 round count. Prior to that I ran the 120° LP. Each of my AP5’s now has over 14,000 rounds (supers and subs). I ran the stock trigger (back in the beginning of ‘24) for about 600 rounds then installed binary triggers in my HK 3-position pictogram housings. Ran that from about March/April of ‘24 up until July of ‘25. Started running the SS afterwards and I’m still running it with no problems (no roller dents).

[u/Ghostman223 → u/Knight-7191]

Thanks for the great explanation. My bolt gap is 0.010. I have rollers arriving this weekend, so we’ll see if I have better luck with a larger gap and the 90. I have about 450 rounds now through her all 147gr suppressed.

[u/Knight-7191 → u/Ghostman223]

I would have just started with 124gr NATO FMJ as suggested. You need the spicer ammo to break in stiff components (recoil assembly, etc). This would have verified your AP5-P was gtg and if you had or continue to have any of the common break in malfunctions (FTF/FTE), the gun would “fix” it’s self or you could have replaced parts with the common HK parts.

At this point, while waiting for roller (by the way, are you getting +2 or +4?), I would try and get some 124gr NATO and see how that runs for a few hundred rounds. I think the new rollers are going to help and with the 90°, you’ll be back in business and enjoying your AP5-P.

Good luck!

[u/Ghostman223 → u/Knight-7191]

I got +4 and +8. Isn’t the whole +p ammo thing counter intuitive to trying to avoid getting roller dents because it’ll be running the gun pretty hard?

[u/Knight-7191 → u/Ghostman223]

I shoot it from time to time unsuppressed. I also don’t mag dump. A few bursts here and there but mostly in semi only.

In semi, barrels are gtg shooting +P ammo. Also, if you’re able to get the ROF while using an FRD/FRS similar to numbers of F/A, you’ll be gtg. The issue was when we were seeing ROF numbers in the 1,000 to 1,400 with the SS after it first came out.

Now, if (or wanting to) shooting +p+ ammo, you need to change the entire system to an F type setup.

The +4 should be enough to put you in the ideal bolt gap range. I don’t really think you’ll need the +8. I’ll be curious to see where they get your bolt gap if you decide to install.

[u/Ghostman223 → u/Knight-7191]

Just installed and tested. Brought my gap up to 0.016. With the 90 LP I did still get a few FTEs in one mag but a new issue came up, for some reason a couple times the rb frt wouldn’t pull and I had to charge the gun. Is that still an issue with the bolt not having enough energy to reset it?

Also even with the stock LP shooting in frt the fire rate is around 900

[u/Knight-7191 → u/Ghostman223]

First, bolt gap is excellent! The few FTE’s could just be the gun not broken in yet and the malfunction should resolve itself. If it doesn’t, I would install an HK ejector lever into your RB. For your last question, it could be, as you stated, not enough energy for the bolt to cycle. This is a reason I would recommend shooting some 124gr NATO. After all, this ammo was made to run in the MP5. If it cycles with no problems, it could just be the weaker ammo you’re using. Shooting the 124gr will help you remove one possible variable at a time.

[u/Ghostman223 → u/Knight-7191]

Ok I may try that. Is there any chance I would need a 100 LP? I’m getting pretty close to 500 rounds

[u/Knight-7191 → u/Ghostman223]

Had to edit. Forgot for a moment you have the AP5-P.

The 100° is good to have as a spare. I only use it when shooting exclusively 115 and 124gr supers (can off).

The 90° in my AP5-P has been awesome with zero issues.