Grendel LMG

[RangerRick]

RPD Belt Photos Part 2

There is a larger gap between the tab on the front and the brass with the Grendel as you would expect. It looks like that tab is used to position the link, not align the bullet. They were pretty beat up by the feed mechanism.



Four Grendel rounds (on right) in a row beside four 7.62 cases.

[RangerRick]

RPD Belt Part 3

Various close up views of the belt.

[RangerRick]

RPD Belt Part 4

This is the connector link between the two 50 round belts. There is a lead belt (on the right in the picture) and a tail belt. The lead belt has the loading tab and ends with part of a disintegrating link.

It looks to me like you could make a whole belt out of these disintegrating links. It wouldn't be as flexible as the wired together links, though.

The tail belt begins with the second part of the disintegrating link, and ends with a small metal tube link. That helps you wind up the belt so it fits in the drum. It probably also dampens whiplash of the last round and locks the bolt open.

I was a little surprised that you couldn't make an infinite belt like with the NATO disintegrating link, but without a quick change barrel, I guess it forces you to let the barrel cool a little while loading a new belt.

Since the RPD is a squad machine gun, it was not meant for sustained auto fire like the bigger guns.







more of this in part 5

[RangerRick]

RPD Belt Part 5

Pictures of assembles lead and tail belt:

[RangerRick]

RPD Belt Part 6

Here is the case head end of the links with both the lead (on right) and tail belts showing . Notice that the tail belt is different from the lead belt. Instead of a detent that fits in the extractor groove, it has plate that is behind the head of the cartridge.

I guess that would prevent the round from getting pushed backwards out of the belt. I don't know if the lead and tail belts are different in all 100 round belts, but I suspect one is a newer design and we just happened to get a mix of the two.





Here is the last link in the tail belt.




The last two pics are of the belt and drum. These 120 FMJ's were loaded to mag length for a Grendel. They had plenty of room. We might have to load to AK mag length (2.205) if we want to have interchangeability. I don't know if the longer rounds will feed, but they fit in the Drum OK.






RR

[stanc]

Here is the case head end of the links with both the lead (on right) and tail belts showing . Notice that the tail belt is different from the lead belt. Instead of a detent that fits in the extractor groove, it has plate that is behind the head of the cartridge.

I guess that would prevent the round from getting pushed backwards out of the belt. I don't know if the lead and tail belts are different in all 100 round belts, but I suspect one is a newer design and we just happened to get a mix of the two.

You're right on both counts. The starter belt is the original Russian design. The other belt is the later Hungarian design, meant to keep rounds from backing out, which tended to happen after belts were reused several times.

The last two pics are of the belt and drum. These 120 FMJ's were loaded to mag length for a Grendel. They had plenty of room. We might have to load to AK mag length (2.205) if we want to have interchangeability.

That's probably unnecessary. When I first brought it up, it had slipped my mind that AKs are made in 5.56x45, which has the same OAL as 6.5 Grendel. My bad.

Looks good, Rick. Ya know, this just might work!

[RangerRick]

That's probably unnecessary. When I first brought it up, it had slipped my mind that AKs are made in 5.56x45, which has the same OAL as 6.5 Grendel. My bad.

Looks good, Rick. Ya know, this just might work!

Yes, I'm very encouraged. The Grendel brass I used was all Lapua fired several times, so it's worst case, since new brass is a little smaller.

There didn't appear to be any warping of the links with the bigger cases. The open portion just opened a tad wider at the front and the closed portion didn't deform at all.

It was a little tighter than the 7.62 brass, but not much. So, I don't think the bolt will have any problem stripping it out of the belt.

This particular belt is a little bent up in places so we'll have to straighten it up a little before use, or get a new manufacture one.

RR

[bwaites]

Ok, I'm a little confused. Take a loose tolerance machine gun and shoot Grendel ammo as just a proof of concept? Is that the idea?

I'll predict that you'll make it work, at least part of the time, and maybe you'll get lucky and it will run great, but to what end? No one is going to put that machine gun back into production because the cartridge works, and from I understand, most of those machine guns have been and are worn out.

If all you are proving is that you can make the Grendel work from links, well, I have no doubt it can be done. Lots of other cartridges work, there is no reason to believe that it won't, simply because it hasn't been done.

I wonder about making it work consistently not so much because of the links, but because the gas system will be far from ideal for the Grendel, the reciprocating mass is not optimized for the cartridge, etc.

We all have pet hobbies and desires though, so I wish you well!

[RangerRick]

If all you are proving is that you can make the Grendel work from links, well, I have no doubt it can be done. Lots of other cartridges work, there is no reason to believe that it won't, simply because it hasn't been done.

I wonder about making it work consistently not so much because of the links, but because the gas system will be far from ideal for the Grendel, the reciprocating mass is not optimized for the cartridge, etc.

The problem is a lot of other people do doubt that it can be done because the cartridge doesn't taper enough, or the shoulder angle is wrong, etc., etc. You can argue with those people forever, but if you can demonstrate a working system, it answers the basic questions.

Also, if you show that it can work in an ancient Soviet belt fed design, and in an AR platform, then the cartridge is flexible enough to be of interest.

As for the reciprocating mass, couldn't the same thing have been said about the AR platform?

[bwaites]

The problem is a lot of other people do doubt that it can be done because the cartridge doesn't taper enough, or the shoulder angle is wrong, etc., etc. You can argue with those people forever, but if you can demonstrate a working system, it answers the basic questions.

Also, if you show that it can work in an ancient Soviet belt fed design, and in an AR platform, then the cartridge is flexible enough to be of interest.

As for the reciprocating mass, couldn't the same thing have been said about the AR platform?

Understand on the doubters!

As for the AR platform, to some extent you are correct. Thus all he research into proper gas port sizes, etc.

The problem is that ancient Soviet belt fed has such loose tolerances you could probably make anything close work at least sometimes!

Good luck, though!

[stanc]

Ok, I'm a little confused. Take a loose tolerance machine gun and shoot Grendel ammo as just a proof of concept? Is that the idea?

Is it just to prove that a 6.5 Grendel machine gun can be built? No.

I'll predict that you'll make it work, at least part of the time, and maybe you'll get lucky and it will run great, but to what end? No one is going to put that machine gun back into production because the cartridge works, and from I understand, most of those machine guns have been and are worn out.

I imagine that's true, but the purpose isn't to show that the RPD can be upgraded to 6.5 Grendel.

I wonder about making it work consistently not so much because of the links, but because the gas system will be far from ideal for the Grendel, the reciprocating mass is not optimized for the cartridge, etc.

Before Bill Alexander began tinkering with the AR15, it wasn't optimized for 6.5 Grendel, either. We can't know for sure if the RPD can be successfully converted until somebody with the necessary knowledge and experience makes the effort.

We all have pet hobbies and desires though, so I wish you well!

It isn't a hobby, pet or otherwise. It's an attempt to achieve a specific goal.

A week and a half ago, Paul opined that the best chance for 6.5 Grendel to become a military cartridge is to interest the V4 nations -- Poland, Hungary, Slovakia and the Czech Republic -- in adopting it, and the "most important factor that would make the Grendel get noticed is if a modern LMG was chambered in it, and shown to provide the same downrange benefits as 7.62 GMPG's." http://www.65grendel.com/forum/showt...ull=1#post9181

Joe suggested, "Let's help him pursue that notion and see where it takes us." http://www.65grendel.com/forum/showt...ull=1#post9236

When nobody else posted any ideas for "pursuing that notion," I started this thread. Since conversion of a modern LMG is not feasible for any of us (if only because there are no disintegrating links available), I proposed using the old RPD as a vehicle that could be used in side-by-side evaluations with OPFOR GPMGs like the PKM.

Paul had previously asked, "Have we already discussed the possibility of using RPD links for the Grendel?" http://www.65grendel.com/forum/showt...ull=1#post9227

Since the Europeans have long been using non-disintegrating links like those in RPD belts, that gave further impetus to the idea of converting an RPD to 6.5 Grendel.

Is it worth doing? I dunno, but it's the only idea I could come up with. If anyone else has alternative proposals that are feasible for us to develop, I'd be more than happy to see them.

[LRRPF52]

The main benefit of an RPD Grendel would be to demonstrate that converting existing systems is doable. I'm not sure if any of the Eastern European Countries are using any RPD variants anymore, since the RPK basically replaced it long ago in the Squad Automatic Rifle role, with 75rd drums for the 7.62x39, and 45rd mags for the RPK-74. Both of the designs don't have a quick barrel change feature, so their role is limited in sustained fire capacity, which is ok for short duration mission profiles such as raids. The RPD was very well-favored by the Rhodesian SAS, due to its compactness and firepower, as no other small belt-fed LMG's existed in the 1960's other than the Stoner 63 System, which they didn't have access to. The RPD is much too costly to produce compared to the RPK, so the USSR dumped it to reserve units as time went on.

Where you might get people's attention with an RPD Grendel is for those who might adopt the cartridge, while a new LMG is being developed. I bet most RPD bores are blown to ****, so a caliber change would not be so out of the budgetary considerations of a small, struggling state. The added benefit of denying your neighbors East & West from using your logistics would be a possible advantage, since many of those countries still use 7.62x39 & 5.45x39 Soviet cartridges.

An RPD/Grendel would probably need gas port tweaking only, since we're looking at the same parent cartridge. It would be fun to see one plugging away at barrier targets to see how belt-fed 6.5 materializes, versus 7.62. I would like to see an improvement to the RPD recoil system, but that's another issue. Something like a mini-M60E4 for the Grendel would be sick, with everything scaled-down, and constant recoil, especially since the M60 is a half-bullpup like the FG42 it was inspired from.

[RangerRick]

An RPD/Grendel would probably need gas port tweaking only, since we're looking at the same parent cartridge.

The RPD also has a 3 position gas block to help tune it.

RR

[LRRPF52]

The 7.62x39 uses a rather large gas port diameter since the bore diameter is large, just as the Grendel uses a larger gas port than 5.56. The pressure differences also drive the gas port diameters, but the op-rod is a simple long-stroke system. Since the RPD has a gas regulator plug, that would be easy to manufacture, in addition to a new barrel. Since the RPD uses the 7.62x39 bolt face, one might only need a new barrel if the existing gas regulator facilitates optimum/functional Grendel gas pressure curves. Here's the 1961 Soviet Manual for the RPD for reference:

http://www.scribd.com/doc/31410672/Soviet-RPD-Light-Machine-Gun-Manual-Degtyaryov-1961

The more I look at the RPD, the more it reminds me of the Bren...gas block, regulator, lower receiver, receiver, op-rod, stock...typical Soviet ingenuity.

[LRRPF52]

Holy crap! Did you notice how cheap the DSA RPD's are? They also have full-auto's for LE/DOD agencies.

http://www.dsarms.com/Semi-Auto-762-X-39-RPD-Rifle--RPDCARBINE/productinfo/RPDCARBINE/

One of their parts kits would be great to start with, then get a Grendel barrel. Pull off that PKM flash hider and do something else of course.

http://www.dsarms.com/RPD%20Products/products/205/