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HK51(308) Improved
Can be built on HK91,SR9,SAR8, Clones, G3 Dealer Samples or Conversions.
Here we can turn your 308 into an 8.3" super compact and controllable fire belcher! Make it even more compact and try one of our strong lightweight aluminum side folders. This little goodie can be set up to fold right or left and has adjustable comb, your choice!

HK53(223) Improved
Can be built on HK93, Clones, HK33 Dealer Sample or Conversions.

Here we can turn your .223 into an 8.3" super compact. With our IMProved model you can add a 22 sub caliber conversion kit, perfect for cheap practice! Try one of our strong lightweight aluminum side folders. This little goodie can be set up to fold right or left and has adjustable comb,
your choice!


The following information applies to both the HK51IMProved and the HK53IMProved in general except where noted.

53surefire.jpgPROFESSIONAL IMProved SERIES
When we say “PROFESSIONAL SERIES” we are referring to the improved version of the HK51/53 series of weapons. Our clients that are in the business of law enforcement, military and special ops need a 100% weapon. We will admit that weapon has yet to be created but we've come close with our IMP improved HK51/53. Our “Improved” modifications also make it OK to use a Choate side folder.. which would beat up other HK51/53’s

This is a Service RDTS has been performing since 1988. It has been very popular with those Individuals who want an HK that is different from the rest and loads of fun to shoot. Also benefiting are those law enforcement professionals who need a very powerful, barricade busting weapon they can bet their life on in an extremely powerful compact configuration.

Compact Firepower
The RDTS HK51/53-IMProved is a compact rifle caliber rifle precision machined to an MP5 configuration (very compact). While we're describing the HK51/53-IMP we need to point out that a Semi-Auto version designated HK51/53-SA is offered for those who want a very individualized (different) weapon than what is offered from the HK factory.

RDTS HK51/53 IMProved Solves the maybe HK51/53 BLUES
In the past we have repaired quite a few HK's, in regard to the 51 style HK's that were done we noticed among other things a persistent problem in regard to being able to reliably chamber, especially aggravated when a full mag was used. The 51 when it was first introduced were very unreliable and according to customers that call us has not been remedied. The problem was caused when the manufacturer cut down the bolt carrier, recoil rod, spring and other items to facilitate the shorter configuration. Also we saw a lack of ability to even eject reliably. A friend who shot one for an article (1990) noted that without the addition of a flash suppressor to increase barrel pressure his would malfunction! This info was not added to the article.

Since a lot of our clients are professional's that depend on their weapons, they could not tolerate this unreliability in an atmosphere that does not allow room for error. RDTS developed modifications for the 51/53 style weapons and started from scratch to produce a weapon that can be used by the serious professional (Law enforcement, SWAT etc.) and recreational machine gunner alike. The result of many man-hours of hands on shop, field and computer work is RDTS's HK51/53-IMP (improved) and in semi the HK51/53-SA .Professional

The procedure that takes your standard configured HK and turns it into a compact reliable HK51/53-IMP is as follows:
We strip the HK down to its basic components and store parts that are not immediately being worked on in its own storage receptacle so as to return all elements to the weapon they originated with.

The barrel is then pressed out and machined on a lathe to an 8.83" length. RDTS does NOT turn down the base of the barrel to ease install, it is the same extremely tight interference fit as came from factory!!!! We then install the barrel back into the receiver with a special support jig that is inserted in the receiver and sets into the receiver extension, that’s the part your barrel is plugged into and also the part that your bolt head rollers lock into…We then use a factory inspired screw type press to SLOWLY press the barrel in till it hits a stop on the jig to insure proper head spacing. We then pull the jig out, install the entire bolt carrier assembly and check head spacing. If it not their yet we put the jig back in till its exactly right. Now the front sight is pressed on and pinned to its new location.

The weapons cocking lever housing tube is then shortened as is the cocking lever support. We join the cocking tube which is ribbed all the way up, as per the factory HK53 to the front forearm band.


We have seen some HK53 conversions that claim to be faithful to the factory with a taper into the front band. Again the factory outside appearance is a cocking tube ribbed all the way to the band that tucks into the front site. The cocking handle support is done in such a way as to not hang loose in the ejection port when the cocking lever is rearward and engaged in the slot catch ready to strip a fresh round out of the mag. A precision mill cuts a new cocking lever slot and catch in the cocking lever housing to facilitate the new configuration. RDTS positions the cocking notch just ahead of the top tube ribs giving a nice clean appearance.

The former cocking handle notch position is welded over and machined as to no longer be visible.



51cock_handle.jpg 51bcocking.jpg

The cocking lever now lies on a stop over part of the front sight, like the HK51 Belted. This enables the 51/53-IMP in conjunction with other modifications to have a very strong and solid chambering stroke and recoil spring pressure.




93_53carrier.jpg 53carrier.jpg

The bolt carrier in our improved HK53-IMP is not cut down in any way! This preserves the weight and balance needed for reliable operation. Another benefit is the recoil spring assembly is only shortened less than 1.5” and very few coils of spring are removed making a very robust positive functioning weapon. And if you have seen the way some 53 carriers have been cut up you could appreciate the uncut carrier even more! The cocking lever support 93_53imp_cocktubes.jpgtube (the cocking lever is attached to the tube that pushes the bolt back) has very little removed from its length as opposed to having to lengthen it. Another big benefit is you won’t have the failure to chamber the first round that happens occasionally on factory HK53 when using accidentally overstuffed 40 round mags. We've been told this never happens but we’ve documented this on a few factory dealer sample guns we’ve had in the shop for coating.

The Bolt Carrier is the next item to be worked on. Due to the compact nature of the HK51-IMP the bolt carrier must be shortened. The 51 carriers that we have seen have a drawback in that when the top carrier tube was cut it was almost all the way back to the main carrier body...basically looking like a stub. The problem now is you have a fairly heavy section of carrier without much of a top section to align it in the receiver promoting wear on the main receiver rail area plus you’ve lightened the mass that used to ram the cartridge into the chamber.

If you inspect one of RDTS's HK51-IMP you'll notice two things:

1. To start with we only remove aprox. 1.5” of length from the top carrier tube. The front top portion of a standard carrier, where the recoil spring passes thru, is a slightly larger diameter, this acts as a guide in the weapons receiver and lets dirt and debris fail behind it onto the smaller diameter of the carrier top guide tube. Our HK51-IMP Bolt Carrier is cut to length on a lathe and the larger diameter section rejoined to the carrier top tube. This is critical to reliable operation in that the larger diameter section has the proper bevel and hardness to let the recoil spring and guide go thru without binding and also prevents premature wear on your receiver that would be caused by a carrier that was simply cut and left with the smaller diameter to rattle around during operation!

2. The recoil spring guide bushing inside the Bolt Carrier is repositioned to optimize the recoil spring length which results in part to the very strong chambering stroke of the 51-IMP. If this was not done the recoil spring would not be able to attain the length needed for reliable operation with the original HK spring, which would result in a heavier spring being used that would also bring the rate of fire up beyond the optimum of 500-600 rpm.


Recoil Spring Assembly
The standard recoil spring assembly on the HK91/G-3 is shortened to operate in the HK51-IMP. RDTS has done extensive testing to find the optimum length but has gone another step beyond!

Listen up, what if you have several HK 308 caliber weapons and want to be able to use your Retractable (A3), Solid (A2) or maybe your 21E stock on your HK91/G-3 AND the HK51-IMP. With the other 51’s this is not possible due to the standard recoil spring and rod being permanently attached anyway, then cut back for use in the shorter weapon it would not then be usable in the full size gun. RDTS has solved this problem, when we build your HK51-IMP we reconfigure your recoil spring assembly to a removable assembly similar to the HK93/M33 or HK94/MP5. For a small charge per additional stock we can modify your other stocks and recoil springs in this manner. So if you have one recoil spring set up for the 51-IMP and another full sized you simply change spring assemblies to whatever stock suits you!

The HK91/93 marking is TIG welded over and re-engraved “HK 53/51“. We need to explain something about the engraving...as of 1998 we use custom plates made from a factory “HK53” dealer sample and computer matched for “HK51”. Also our method uses electrolyze etching which means no dents on your weapon. The engraving is sharp, crisp and indistinguishable from the factory engraving!!! RDTS engraving has always been very good, now its factory perfect.

Our own RDTS factory markings have since 1991 been positioned at the rear of the receiver by the stock pin holes and are concealed by the stock.



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