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Posted: 9/15/2026 9:07:28 PM EDT
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I've spent more on gun builds by a wide margin, my total cost to build my Mk18 clone was north of 7 grand. But for some reason I always clam up at the idea of buying a gun, all in one go, that costs more than about a thousand bucks. I finally talked myself into it today, even if it's a lesser clone. This has been a grail gun for years now, and it's my hope to ensure I have an MP5 in good working order in time for the AS Designs FRT pack coming out this year. I'm not a newbie to guns, but the vast majority of my experience is in AR15s. I own a PTR91 GI, which I like, but that's the extent of my roller lock knowledge. I've absorbed a fair amount of ancillary information about MP5s over at least a decade, but nothing compares to actually handling and shooting a gun to learn its quirks. I intend to shoot it almost entirely suppressed, most likely with my YHM R9 on a Tri Lug mount. I believe there are locking pieces labeled in degrees that should be selected for this use case, but I don't know what brand of locking piece is good, or which one to select. I'll be SBRing it as soon as practicable, though I may wait on the actual assembly as an SBR until there's more clarity about the engraving requirements. Until then it will have an HKPDW brace that ended up in my collection, though I don't know how it did. I am very excited. Specifically it's the 8.9" model, with what I call "Cold War" forend. That's the classic look that feels right to me. https://atlanticfirearms.com/century-arms-ap5-core |
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You'll love it!  Originally Posted By ELittlefield: I am very excited. Specifically it's the 8.9" model, with what I call "Cold War" forend. That's the classic look that feels right to me. Later HK started offering the Tropical (or wide) forend. Then the Surefire 628 became all the rage. |
| Generally, you don’t need to swap locking pieces for running suppressed on the full sized guns, unlike the K guns. The 100 degree LP is fine for both suppressed and unsuppressed. I don’t what the AP5s come with, but the HK SP5s come with the 100 degree piece. If you need to swap LPs, the HK factory ones are the best. RCM is also good. |
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Originally Posted By KitBuilder: You'll love it! Yeah, that's the original forend. HK was still selling them like that when Army SF first bought some. Later HK started offering the Tropical (or wide) forend. Then the Surefire 628 became all the rage.  |
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Originally Posted By willi3d: Generally, you don’t need to swap locking pieces for running suppressed on the full sized guns, unlike the K guns. The 100 degree LP is fine for both suppressed and unsuppressed. I don’t what the AP5s come with, but the HK SP5s come with the 100 degree piece. If you need to swap LPs, the HK factory ones are the best. RCM is also good. Very good, thank you. |
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Originally Posted By ELittlefield: Very good, thank you. The general consensus is that MKE has been shipping AP5s with 120 degree locking pieces in an effort to minimize malfunctions caused by people shooting stockless pistols. Based on that, I’d probably install a 100 or even a 90. Rim Country Manufacturing seems to be a preferred source. They’ve been hard to find lately, check AS Designs. When I bought mine, a “10%off next purchase” card was in the box. Hopefully they honor that on the ARHK. |
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I have the AP5 core. Form 1'd it twice. Once for my first trust but never "made". Then voided the F1 and refiled under a new trust. I have zero care for any traditional look and bought a JMAC picatiny stock adapter for it. I have a Sig buffer tube folding stock with a SBA3 for now as I am also waiting for the final ATF rule in engraving. If you have any care to follow 922R, then the ARKH should cover some of the required US made items. I bought a B&T optic mount. Put a spare EPS on it I had. I replaced the forend with the tapered one. Want the MI one someday. I bought a Lee lower becasue the trigger is terrible on the AP5. Then FRT's became popular. The Arc-fire doesnt work with the Lee. A super safety does. But you are getting the ARKH and so moot. I shoot 147g reloads and use a Bixler Tri-lug mount on a R9. Works great. I also installed a HK ejector and some spare extractor springs but havent need to replace yet. I also bought a 90 and a 80 deg LP and am running the 90 for now. Bought a couple korean mags. Meh. Tight but they work. Got a couple real HK ones from Lanbo's. MKE ones have been good too. Paint scratches easily on mine. Smooth shooter. |
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Originally Posted By osprey21: I'm in the process of replicating this late 70's MP5 forend with removable hand stop. https://www.ar15.com/media/mediaFiles/766/Late_70_s__MP5_-_Copy-3827963.png Nicely done! 
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@ELittlefield The AP5 that you bought has an “SEF” selector lower (in HK speak, the “trigger housing”). My understanding is that the ARHK will ship initially only with an “Ambi” selector. Dumb as it sounds, the selector axle holes and markings are not the same (SEF holes are smaller, markings don’t align well). Typical German engineering. Depending on the diameter of the selector axle ASD use, ARHK with Ambi selector may or may not fit your original SEF lower. If not, you could buy a Ambi lower ($$$) or try the Magpul. The easy button would be to get the SEF version of the Rare Breed FRT instead of the ARHK, but I can understand why many folks would prefer the ASD version. |
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Originally Posted By StealthyBlagga: @ELittlefield The AP5 they you bought has an “SEF” selector lower (in HK speak, the “trigger housing”). My understanding is that the ARHK will ship initially only with an “Ambi” selector. Dumb as it sounds, the selector axle holes and markings are not the same (SEF holes are smaller, markings don’t align well). Typical German engineering. Depending on the diameter of the selector axle ASD use, ARHK with Ambi selector may or may not fit your original SEF lower. If not, you could buy a Ambi lower ($$$) or try the Magpul. The easy button would be to get the SEF version of the Rare Breed FRT instead of the ARHK, but I can understand why many folks would prefer the ASD version. Thank you for that update. I will take that into account. |
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Originally Posted By KitBuilder: Yeah, you're right, that is the actual original forend. Any idea how many of those left the factory on production models? (Google thinks a few thousand, but says it's not sure.) Nicely done! Originally Posted By KitBuilder: Originally Posted By osprey21: I'm in the process of replicating this late 70's MP5 forend with removable hand stop. https://www.ar15.com/media/mediaFiles/766/Late_70_s__MP5_-_Copy-3827963.png Nicely done! When I was a young kid that’s how they looked back then. I always thought they looked wrong with the wide forend added later. After I Finally picked up one of my very own, a SP5 a few years ago I added the old school HK one I had after I got home with it. Had a couple stashed away for just such an occasion. One is a IC date code and the other is a IF code. I’m a bit of a fudd for the old school looks and memory. |
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Originally Posted By StealthyBlagga: @ELittlefield The AP5 they you bought has an “SEF” selector lower (in HK speak, the “trigger housing”). My understanding is that the ARHK will ship initially only with an “Ambi” selector. Dumb as it sounds, the selector axle holes and markings are not the same (SEF holes are smaller, markings don’t align well). Typical German engineering. Depending on the diameter of the selector axle ASD use, ARHK with Ambi selector may or may not fit your original SEF lower. If not, you could buy a Ambi lower ($$$) or try the Magpul. The easy button would be to get the SEF version of the Rare Breed FRT instead of the ARHK, but I can understand why many folks would prefer the ASD version. @StealthyBlagga what kind of lowers are expected to look correct with the ARHK? |
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You'll likely be quite pleased with it. I've had both HKs and MKEs, and at least as far as my samples, the MKE is every bit as solid when it comes to function and reliability. The fit and finish on the MKE isn't as nice, but as to whether that's worth the accompanying $2k premium for the HK is... highly dependent on your priorities, lol. In my experience, the MKE guns come with a very roughly machined locking piece with a vague and unknown angle -- somewhere between 115 and 120 degrees. I'd recommend just springing for an RCM 100-degree for the full-size. Considering the cost of everything else with an HK-related build, it's sort of a rounding error. |
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Want To Buy: Savannah Mk18 RIS II in light blonde/bronze - LMT SOPMOD Gen 1 Tan - Geissele S3G
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Want To Buy: Savannah Mk18 RIS II in light blonde/bronze - LMT SOPMOD Gen 1 Tan - Geissele S3G
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Originally Posted By krdt: You'll likely be quite pleased with it. I've had both HKs and MKEs, and at least as far as my samples, the MKE is every bit as solid when it comes to function and reliability. The fit and finish on the MKE isn't as nice, but as to whether that's worth the accompanying $2k premium for the HK is... highly dependent on your priorities, lol. In my experience, the MKE guns come with a very roughly machined locking piece with a vague and unknown angle -- somewhere between 115 and 120 degrees. I'd recommend just springing for an RCM 100-degree for the full-size. Considering the cost of everything else with an HK-related build, it's sort of a rounding error. Thank you. |
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Agree on the 100° LP and RCM currently has them in stock. $40.00 |
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Nice, I'll get that ordered. |
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Originally Posted By ATLANTIC-FIREARMS: https://atlanticfirearms.com/410748/century-arms-ap5-core.jpg Nice. |
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Originally Posted By ELittlefield: what kind of lowers are expected to look correct with the ARHK? @ELittlefield It's complicated. From what I have seen, the ARHK looks to have been designed primarily for compatibility with the HK SP5, which has a Navy Ambi-style trigger housing (lower). There are really two issues: 1) The holes for the selector levers on your SEF trigger housing may or may not work. My Rare Breed Ambi FRT selector levers would not sit properly in the smaller holes of my SEF trigger housing, nor in those of a Magpul SL Grip Module (which is an odd hybrid of SEF and Ambi). I managed to make the Magpul SL Grip Module work by enlarging the holes with a drill press and Dremel and now it is functional. The upside is that the Magpul SL is cheap enough that you won't feel bad modifying it. 2) The selector markings on the SEF and Ambi (pictogram) trigger housings are quite different. The ARHK selector will move to three positions and you will know what they are, but the line on the selector lever won't align properly with the SEF markings or Magpul SL pictograms. My comments about ARHK compatibility are really guesswork as we don't have the needed details from AS Designs yet. I am basing this on my experience making my Rare Breed Ambi trigger work in a Magpul SL trigger housing on my PTR 9CT. After getting it to work, I subsequently moved the Rare Breed to an AP5-SD, which has 4-position Ambi pictogram markings... the selector line still doesn't align properly, falling mid-way between the "burst" and "auto" pictograms, but again it is workable. If you could find just a MKE Navy trigger housing, that would be a good option, but I don't see them on Atlantic's website. If you are anal about fit and appearance, you could look at using an HK 3-position trigger housing, but the price of this one is outrageous and I am not sure what (if anything) you would have to modify for it to work on your AP5. Next best might be to get an HK SP5 trigger housing and have it laser engraved with the full-auto pictogram, though I am not sure if it is a drop-in fit with the AP5. Lastly, you could do what I did and try the Magpul SL, assuming you are willing to drill a ~15.4mm (39/64") hole in it. Now you know why I recommended getting the Rare Breed SEF FRT for that gun 
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