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A Basic Guide to Roller Lock Variants

(photos in this article have been provided with permission by Dakota Tactical unless otherwise noted)

Over the years I've seen a lot of variations on the basic roller delayed blow back system (commonly referred to as "roller lock" in spite of there being no actualy "locking" involved) like the HK MP5 family of weapons but haven't ever found what I consider to be a solid guide to them all. In this article I'm going to do my best to remedy that so let's get started.

A Little Family History

I could write for days on the history of the platform but I'd just bore 99% of you to tears so we'll do it short and quick. German engineer Ludwig Vorgrimler was recruited in by Mauser Werke working in arms development from 1936 to 1945. After the war ended blah blah blah which brings us to his move to Madrid in 1950 to work for CETME where he began developing a roller delayed rifle based off the Nazi German STG45 (no not the STG44 but we could do a biblical list of "and lo begats" here tracing lineage) which would become the CETME Modelo A in 762CETME (basically powder puff 762NATO). Because the German border guards wouldn't accept a non NATO caliber the rifle was retooled for 762NATO and dubbed Modelo B.

In 1952 West Germany licensed the CETME design to be put into production by Heckler & Koch as the G3 (and there arose a great wailing, and rending of clothes, and gnashing of teeth from the H&Kool-Aid Kult.......sorry gang the modern "roller lock" is a SPANISH design). This eventually would lead to a pretty impressive array of weapons configurations. So.......

The Four Basic Food Groups

Roller delayed guns come in 4 main calibers (762x51 762x39 556x45 and 9mm) and a variety of lengths that very often borrows from other members of the family tree. Trouble is.....they all use numerical designations that don't really tell us anything useful most of the time. Almost all of these weapons, without regards to caliber, are capable of using fixed and retractable stock and a few have side folding ones as well as trigger packs in semi auto, select fire, and burst fire. We won't be discussing stocks in depth (A2/A3) or trigger packs (A4/A5/ETC) because it would just make this guide longer than it really needs to be.

We're also going to ignore a lot of rare or exotic variants because I'm lazy and most of you aren't going to notice or care. Please note that not all models that will be listed here will be official HK configurations but have become at least common enough within the market in some form or another that you might run across it on the web.

Let's work through them from largest to smallest.


PSG-1 - semi auto only "sniper" variant with free floated 25.5" barrel and other upgrades

G3/HK91 - rifle length barrel 16" or longer with 16" 18" and 20" being common. Something most people don't realize is that the G3 and HK91 are essentially the same rifle. The G3 is the full auto battle rifle variant and the HK91 is the commercial semi auto version.
(Yes I KNOW there was a 41 but the 91 replaced it. No I'm not reconsidering my decision to ignore irrelevant variants. Go away.)

G3K - 12.4" barrel
(Image from ARFCOM of Dakota Tactical build)

HK51 - 8.375" barrel and uses MP5 hand guards (51/53 guards exist and are a little longer at the back to cover the trunnion a little more)

HK51K - 5.3" barrel and uses MP5K hand guards (SP89 guards would work but the blast from the flash hider might damage it)
(Image from Google)


HK32 - rifle length barrel 16" or longer (I am unaware of any other length than 16" in actual existence)
(Image from Google)

HK32K - 12.7" barrel. Especially observant readers (yes both of you) will very likely notice the magazine difference between this and the other two variants pictured here........PTR manufactures the 32 series to be compatible with AK47 magazines. HK developed the original with their own mags but for all intents and purposes the HK32 does not exist. Go get yourself a PTR and a bag full of cheap AK mags and live life to the fullest. You know you want one......god knows I do which brings us to the....
(Image from HKPRO)

HK32PDW - 9" barrel uses MP5 handguards
(Image from Google)


PSG-1 - An uncommon variant with 26" barrel based off the original PSG-1 with all the associated changes
(See also PSG1)

HK33/93 - rifle length barrel 16" or longer
(Image from HKPRO)

HK33K - 12.69" barrel

HK53 - 8.25" barrel and uses MP5 handguards.

HK53K - 5.3" barrel and uses MP5K handguards
(Image from HKPro)


HK94 - rifle length barrel 16" or longer. DOES look absolutely ridiculous.
(Image from Google)

HK34 - rifle length barrel 16" or longer but uses rifle length hanguards. This is a bizarre American bastard child and precisely what the HK94 should have been. The Koch dropped the hell out of that ball. One day the Pariah stables will be home to one of these. I'm looking suggestively in your direction Dakota Tactical.

MP5 - 8.85" barrel. Yes this is the one we ALL know and love.

MP5SD - uses a modified receiver, ported 5.75" barrel, and custom silencer to reduce the velocity of supersonic ammunition to subsonic velocities and suppress muzzle report.

MP5K/SP89 - 5.85" barrel and shortened receiver. Not compatible with retracting stocks. The MP5K and SP89 do not feature a 3 lugged barrel or buttstock. Those features were introduced with the MP5K-PDW. SP89 is the semi auto commerical variant, MP5K is the full auto military variant.

MP5K Reverse Stretch - 5.85" barrel with full sized receiver for use with retracting stocks while retaining the short front end of the true K variant.

And Now For The Odd Ducks And Belt Feds


HK21 - Belt fed variant
(Image from Google)


HK23 - Belt fed variant
(Image from Google)

.40 Smith and Wesson

MP5-40 - Just like the original but chambered in 40SW with a modified receiver to acommodate the new stick mags AND the addition of a last round bolt hold open feature at the behest of the US federal government. This model originated as the MP5-10 but due to the low adoption rate of the 10mm and subsequent interest in 40SW, along with the similarities between the two cartridges, this was basically a no brainer.

10mm Auto

MP5-10 - Just like the original but chambered in the incredibly powerful 10mm at the request of the US FBI a last round bolt hold open feature was added and the receiver modified in order to accept new stick mags for the caliber. Due to the low adoption rate of 10mm and subsequent development of the 40SW using 10mm as the parent brass converting the design over to the new caliber required almost no work.

7.62x35 AAC 300 Blackout

Anything that is chambered in 556 can theoretically be done in 300BLK. Common variants being the 93/53.


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