Grendel as a Universal Infantry Cartridge

[Tony Williams]

And the SPC guys are getting close with 6.8x43 loaded with long-ogive bullets, although it sounds like chamber pressure may be on the high side. Also, they are talking about making a "stretched" 6.8x46, a la Tony Williams' proposal.

My thinking on this subject has been gradually evolving. I'm now inclined to think that while 6.8x46 may be about right, the case diameter should be wider than the 6.8x43, for two reasons: the need to accommodate significantly longer copper+steel bullets, especially tracers, which will occupy more of the case capacity; and the desirability of allowing for the future use of part-polymer cases, which may have thicker walls than metal ones, thereby further reducing propellant capacity.

I think it is essential for the success of the concept that soldiers using the new round in combat must not notice any shortfall in comparison with the 7.62mm, especially in long-range hit probability, otherwise they will still insist on keeping the 7.62mm. If the new round can provide this while achieving significant reductions in weight and recoil, then I think it could kill off the 5.56mm, given that many soldiers have said that they would rather carry 7.62mm weapons than 5.56mm.

[Tony Williams]

A 6.5 or 7mm DMR/Sniper SASS with significantly less weight than an M110 would be great, but in the end...what performance advantages will we gain over a 5.56 SPR with 18" pipe pushing 77gr SMK's? It would have to be significant enough to overcome the benefits the SPR currently has with ammo compatibility, lightweight, and mag capacity-per-volume.

But the big army will never adopt anything like the SMKs, which have a plain lead core (and are far too expensive anyway). What they want is shown by the M855A1 EPR - a combination of a steel and copper core: they're working on an M80A1 along similar lines. These will be much longer than lead-cored bullets, and tracer versions will be longer still. They will also need a cannelure for belted MG use, further affecting the BC. Since a long effective range will be required to replace 7.62mm (800-1000m), this pushes the optimum calibre and case size upwards.

[JASmith]

But the big army will never adopt anything like the SMKs...

Certainly correct...

The challenge, however, is that detractors of the change we're discussing will use the Mk262 as a standard, the same way the Hornady 75 gr SuperFormance round will be used as a standard.

These specious and somewhat emotional arguments exist and influence the perceptions of even the most astute decision makers.

Patient education is one path. Raising our threshold performance is another. May need to do both...

[stanc]

My thinking on this subject has been gradually evolving. I'm now inclined to think that while 6.8x46 may be about right, the case diameter should be wider than the 6.8x43, for two reasons: the need to accommodate significantly longer copper+steel bullets, especially tracers, which will occupy more of the case capacity; and the desirability of allowing for the future use of part-polymer cases, which may have thicker walls than metal ones, thereby further reducing propellant capacity.

Okay, to meet all of those needs it looks like the .445" diameter case of the 7x46 UIAC is what will be required. (Possibly a .430" case could do the job, but I don't know of any viable parent case with that diameter.)

I think it is essential for the success of the concept that soldiers using the new round in combat must not notice any shortfall in comparison with the 7.62mm, especially in long-range hit probability, otherwise they will still insist on keeping the 7.62mm. If the new round can provide this while achieving significant reductions in weight and recoil, then I think it could kill off the 5.56mm, given that many soldiers have said that they would rather carry 7.62mm weapons than 5.56mm.

The available info on the 7mm UIAC indicates that trajectory and wind drift are comparable to, if not better than, that of 7.62mm, which seems to address your hit probability concern.

Felt recoil of 7mm UIAC ought to be noticeably less than 7.62mm, and my calculations show a 25% weight reduction relative to 7.62mm (which I'd rate as significant).

[stanc]

But the big army will never adopt anything like the SMKs, which have a plain lead core (and are far too expensive anyway). What they want is shown by the M855A1 EPR - a combination of a steel and copper core: they're working on an M80A1 along similar lines. These will be much longer than lead-cored bullets, and tracer versions will be longer still. They will also need a cannelure for belted MG use, further affecting the BC. Since a long effective range will be required to replace 7.62mm (800-1000m), this pushes the optimum calibre and case size upwards.

I agree that SMKs won't be adopted for general issue, because of the reasons you noted. However, even with the requirement for a "green" bullet in the general purpose round, for the near term at least, it seems likely they'll continue to use SMK loads for snipers.

[stanc]

The challenge, however, is that detractors of the change we're discussing will use the Mk262 as a standard...

I think that can best be answered by comparing 5.56 Mk262 and 7.62 M118LR to a MatchKing load in the new round.

We just have to make sure that all comparisons are like to like. SMK vs SMK, FMJ vs FMJ, etc, but no SMK vs FMJ arguments.

[Tony Williams]

I agree that SMKs won't be adopted for general issue, because of the reasons you noted. However, even with the requirement for a "green" bullet in the general purpose round, for the near term at least, it seems likely they'll continue to use SMK loads for snipers.

Oh certainly - I expect that to continue for the foreseeable future. Too few of these rounds are fired for lead pollution to be an issue even for the most die-hard Green.

[stanc]

My thinking on this subject has been gradually evolving. I'm now inclined to think that while 6.8x46 may be about right, the case diameter should be wider than the 6.8x43, for two reasons: the need to accommodate significantly longer copper+steel bullets, especially tracers, which will occupy more of the case capacity; and the desirability of allowing for the future use of part-polymer cases, which may have thicker walls than metal ones, thereby further reducing propellant capacity.

Okay, to meet all of those needs it looks like the .445" diameter case of the 7x46 UIAC is what will be required.

Next question: Is it better to just push for 7mm UIAC, since that cartridge (and a 7x46 GPMG!) already has been developed and there's some data for it? Or is it worth spending the time (and money?) to "study" the possibilities of 6.8 and 6.5 versions?

Since the 7x46 info says (with a 130gr bullet) it has 7.62x51 muzzle velocity, is it likely that a 6.8x46 w/120gr bullet and 6.5x46 w/110gr bullet would have approx the same MV? Or would 6.8 and 6.5 loads need even lighter projectiles to match MV?

[patdaly]

Adding an op-rod driven system to the AR only makes it worse, not better. That has been the result time and again with the HK416, POF, ARES kits, etc.

Dumb question, has ADCORs system been evaluated?

https://www.adcordefense.com

Seems pretty elegant.

[Tony Williams]

Next question: Is it better to just push for 7mm UIAC, since that cartridge (and a 7x46 GPMG!) already has been developed and there's some data for it? Or is it worth spending the time (and money?) to "study" the possibilities of 6.8 and 6.5 versions?

Since the 7x46 info says (with a 130gr bullet) it has 7.62x51 muzzle velocity, is it likely that a 6.8x46 w/120gr bullet and 6.5x46 w/110gr bullet would have approx the same MV? Or would 6.8 and 6.5 loads need even lighter projectiles to match MV?

It's a matter of which compromise strikes the best balance of advantages. I don't think there's a lot it in, but given that IMO 6.5mm is the minimum to do the job and 7mm the maximum, I now tend towards 6.8mm.

[warped]

I agree that SMKs won't be adopted for general issue, because of the reasons you noted. However, even with the requirement for a "green" bullet in the general purpose round, for the near term at least, it seems likely they'll continue to use SMK loads for snipers.

They would be better off using Scenars or AMAX, More velocity and flatter shooting at range. Much higher BC

[LRRPF52]

Dumb question, has ADCORs system been evaluated?

https://www.adcordefense.com

Seems pretty elegant.

The problems I see with all op-rod driven systems are that you now have more weight in the front of the gun, which affects center of balance and how a soldier or user interfaces with the weapon's center of gravity. Next, you now have a critical moving part that is no longer contained or shielded within the receiver, but quite exposed to the elements, to include large sand particles and debris.

Every external piston system I have used and carried in the field requires pretty attentive maintenance to prevent rust and carbon residue from affecting operation of the weapon. In recoil, any external piston system will present off-bore axis reciprocating mass that usually induces muzzle rise and pronounced felt recoil.

The genius in Stoner's design is that the center of balance is above your hand, recoil is straight from muzzle to shoulder, and there are absolutely zero moving parts of the action outside the receiver. He basically took the piston, and combined it with the bolt inside the bolt carrier to reduce weight, protect the mechanism, and make recoiling parts move in-axis with the bore. No other operating system that I know of handles as well, unless you use a constant recoil principle.

My opinion is that the external op-rod systems for the AR15 are a fad that several people have chosen to jump onto due to market demand, which has been fueled by various gun rags claiming the inferiority of the direct impingement system crapping on itself. Carbon fouling is not an issue with DI AR's, but lack of lubrication is. It's basically a single piston combustion engine. Use quality engine components, good lubrication, and the correct gas, and it will run fine.

[warped]

Yep and a proper coating of GOOD lube on the bolt tail keeps crap from sticking, heck I polish the bolt tail.

[LRRPF52]

One of the only lightweight op-rod driven AR's I have felt is LMT's offering, which I handled quite a bit at SHOT in 2008 or 2010. I saw another company that was using a valve with a tiny valve spring in an op-rod, which is probably the worst idea ever. Imagine a little valve spring right near the gas block, that will see temps up to 500 degrees F...

If I remember right, that was Osprey Defense, who also claim a 10,000 round MBTF (Mean Time between Failure). They have been known to rig tests to make their system look superior to a DI system, including water submersion with duct tape covering the suppressor opening, which would have the same results with a DI gun.

I recently handled some LWRC M6's, and they were really heavy compared to a similar profiled DI gun.

A friend of mine who has been selling his own op-rod system just made a drop-in DI conversion for it, which I was joking about last week, tongue-in-cheek. I didn't think anyone would actually do it. He did do something that addresses another issue with the DI system's gas tube, which I am really interested in now. I'll have to check into it some more.

[Tony Williams]

Personally, I'm not bothered about which operating system is used as long as it works.

I recently listed my basic requirements of any military small arm as follows: reliability, robustness, reliability, good ergonomics, reliability, easy maintenance, reliability, ability to accept a wide range of accessories, and of course reliability in extended combat conditions...