Optimum 6.5 Grendel bullet weights

[stanc]

Since there are so few Grendel-specific threads in the section provided for military use of 6.5 Grendel, I thought I start one.



What do you think are the optimum weights for military loadings of 6.5 Grendel?

I suppose it would depend on whether or not 6.5 Grendel is viewed as a potential replacement for both 5.56 and 7.62 NATO, or just 5.56 NATO alone.

BALL

The general purpose round, used in carbines, rifles, machine guns. In 5.56 NATO, a 62gr projectile is standard. Some people consider 110 grains to be "just right" for a 6.5 ball load. What weight do you think is best for 6.5 Grendel, and why?

SMK

A special purpose bullet, primarily used in sniper rifles. In the 5.56 Mk262 load, 77gr is the projectile weight. The only 6.5 SMK factory load currently on the market has a 123gr bullet. What weight SMK do you think would be best for 6.5 Grendel, and why?

TSX

Another special purpose bullet, mainly used in the 10.5" barrel, Mk18 CQB carbine. In the 5.56 "brown tip" load, 70gr is the bullet weight. The only 6.5 TSX factory load currently on the market has a 120gr projectile. What weight TSX do you think would be best for 6.5 Grendel, and why?

P.S. Hypothetical weights are permissible for this discussion.

[JASmith]

BALL
[img]...In 5.56 NATO, a 62gr projectile is standard. Some people consider 110 grains to be "just right" for a 6.5 ball load. What weight do you think is best for 6.5 Grendel, and why?

Look at 95 -100 gr bullets with AMAX shape -- they seem to come closest to the M855 trajectory out to 500 meters.

SMK
[img...In the 5.56 Mk262 load, 77gr is the projectile weight. The only 6.5 SMK factory load currently on the market has a 123gr bullet. What weight SMK do you think would be best for 6.5 Grendel, and why?

Can go heavier terminal performance and drift resistance more important for what appears to be a sniper mission -- The 140 gr MK likely too heavy. Consider the 130 gr Berger VLD (high ballistic coefficient and similar construction to MK, so might be allowable under JAG guidelines)

TSX
[img]...Another special purpose bullet, mainly used in the 10.5" barrel, Mk18 CQB carbine. In the 5.56 "brown tip" load, 70gr is the bullet weight. The only 6.5 TSX factory load currently on the market has a 120gr projectile. What weight TSX do you think would be best for 6.5 Grendel, and why?

Any weight of 100 gr or better will better the SD of the 70 gr .24" bullet. The 120 gr TSX would do just fine in this role - provided JAG allowable.

[RangerRick]

High energy and high Ballistic Coefficient (BC) are desirable. Longer and heavier gives high BC allowing the bullet to retain energy at longer ranges. Lighter and faster gives higher energy. Since energy increases as the square of increasing velocity and momentum increases linearly, you get more by increasing velocity than you lose by reducing weight. As far as muzzle energy is concerned anyway.

Reducing weight in a military bullet of a given caliber that ideally can't have hollow cavities for for legal and barrier penetration reasons usually means a shorter bullet. shorter and lighter means lower BC's.

If we assume the military is going to want an M855A1 type bullet with a copper core and steel penetrator, then the length can be preserved as compared to a lead core bullet without voids.

We know that the Grendel starts to lose energy at above 120 grains or so grains of bullet weight because of powder, length, and pressure limits of the cartridge.

Since those factors are trade offs, plotted lines of Energy and BC on a graph as the bullet weight decreases would cross at some bullet weight. That would be the optimum, and it looks like that point is at about 110-115 grains.

If we look at designs with the steel penetrator tips, in addition to making the bullet longer, the steel tip is lighter than the copper/zinc alloys used in gilding metal. That shifts the center of gravity to the rear, which makes it more likely that the bullet will tumble on impact. That's a good thing for stopping ability.

I get about 2725 feet per second out of 107 grain bullets from a 20 inch barrel at less than a max load, so 2725 fps from a 110 bullet is doable. You should be able to get a BC of around .450 from a bullet like that.

That yields 1814 foot-pounds of energy at the muzzle compared to the 1742 ft-lbs I get with 123 grain bullets at 2525 fps in the same 20 inch tube. They have about the same energy at 275 yards with the 123 grain leading from there on out, if you believe the ballistic calculators.

If the 110 grain bullet could get it's BC up to .500 it would beat the 123 out to 325 yards. It may be possible, but I doubt it. The calculator I have shows the Barnes 110 grain bullet as beating the 123 Scenar in energy only out to 200 yards.

For a hypothetical 115 grain 6.5 mm with a .500 BC bullet at 2650 fps from the same tube, I get 1794 foot-pounds at the muzzle and there again about the same as the 123 Scenar at 300 yards.

So I guess it depends on what's more important to you, performance under 300 yards or over. The 115 drops to 500 foot-pounds at about 875 yards, and the 110 grain bullet at about 800 yards. The 123 grain bullet makes it to 925 yards with 500 foot-pounds. There again if you believe the calculators.

This is kind of a worst case comparison because I doubt you could make and M855A1 type bullet with a copper core and steel penetrator at a weight of 120-123 grains with a BC matching the Lapua Scenar and not have it so long that it reduces the powder load.

If you can get a 110 grain bullet up to .500 BC, it is going to have superior BC out past 1000 yards. With a more realistic BC 0f .475 the 110 grain bullet will be surpassed by the 115 at about 300 yards and will drop to 500 foot-pounds at about 840 yards.

Rick

[stanc]

If we assume the military is going to want an M855A1 type bullet with a copper core and steel penetrator, then the length can be preserved as compared to a lead core bullet without voids.

We know that the Grendel starts to lose energy at above 120 grains or so grains of bullet weight because of powder, length, and pressure limits of the cartridge.

Since those factors are trade offs, plotted lines of Energy and BC on a graph as the bullet weight decreases would cross at some bullet weight. That would be the optimum, and it looks like that point is at about 110-115 grains.

Interesting. Thanks!

If we look at designs with the steel penetrator tips, in addition to making the bullet longer, the steel tip is lighter than the copper/zinc alloys used in gilding metal. That shifts the center of gravity to the rear, which makes it more likely that the bullet will tumble on impact. That's a good thing for stopping ability.

This is kind of a worst case comparison because I doubt you could make and M855A1 type bullet with a copper core and steel penetrator at a weight of 120-123 grains with a BC matching the Lapua Scenar and not have it so long that it reduces the powder load.

Concur. It looks to me like it'd have to be at least as long as the 120gr GMX, which has a BC much lower than the 123gr Scenar.

[stanc]

TSX

Any weight of 100 gr or better will better the SD of the 70 gr .24" bullet. The 120 gr TSX would do just fine in this role - provided JAG allowable.

Since the 5.56mm 70gr TSX is JAG-approved, I'm sure there'd be no problem in that regard with 6.5mm TSX loads.

But, I'm not so sure the AA 120gr factory load is a good choice. From a 10.5" barrel, MV would likely be no more than ~2175 fps. Minimum expansion velocity of the 120gr TSX is 1800 fps.

For use in the Mk18, it looks to me like a much lighter TSX -- say, 85-100gr -- would be optimal, in that MV would be high enough to ensure maximum expansion in soft tissue.

[JASmith]

...From a 10.5" barrel, MV would likely be no more than ~2175 fps. Minimum expansion velocity of the 120gr TSX is 1800 fps...

The 2175 figure might be on the high side unless the powder is selected for optimal performance in the short barrel. A load using TAC to get 2420 from a 24" barrel would be likely to be somewhere between 2025 and 2125 ft/sec per the Ammoguide.com velocity estimator.

Going lighter means you give up penetration depth and barrier defeat. Bottom line, it depends on what effects you want to emphasize.

[stanc]

The 2175 figure might be on the high side unless the powder is selected for optimal performance in the short barrel. A load using TAC to get 2420 from a 24" barrel would be likely to be somewhere between 2025 and 2125 ft/sec per the Ammoguide.com velocity estimator.

That's quite possible. 2175 fps was an estimate, based on the figures in http://www.alexanderarms.com/item/12...ammunition.htm and http://www.alexanderarms.com/grendel_ballistics.pdf

Going lighter means you give up penetration depth and barrier defeat.

IMO, you wouldn't give up anything of consequence. Lightweight TSX bullets should be quite able to defeat windshield glass and car doors just as well as heavier TSXs.

And judging by 6.8 SPC 85gr TSX gel tests, penetration depth appears to be more than adequate, with no significant loss compared to the 110gr TSX.



For use in a 10.5" barrel carbine, going lighter seems to be all plus, with no minus worth worrying about.

[JASmith]

...And judging by 6.8 SPC 85gr TSX gel tests, penetration depth appears to be more than adequate, with no significant loss compared to the 110gr TSX.

The more meaningful gel test would be one with a barrier, e. g., body armor, windshield glass, door, etc. between the rifle and gel.

The TSX and TTSX are good hunting bullets -- but would like to see tests relevant to Military or Law-enforcement needs.

We can talk about optimum weights for short barrels after seeing that data or a credible ballistics analysis.

In any event, the CQB round mentioned in the OP was a 70 gr "Brown Tip". We almost certainly want to at least match, if not exceed, the sectional density of that bullet. That would result in a minimum bullet weight of 97 grains.

[stanc]

The more meaningful gel test would be one with a barrier, e. g., body armor, windshield glass, door, etc. between the rifle and gel.

Why would that be "more meaningful"??? In CQB, how often is there a barrier between shooter and target?

The TSX and TTSX are good hunting bullets -- but would like to see tests relevant to Military or Law-enforcement needs.

If a bare gel test is not relevant to those needs, why is it done in military and law enforcement tests?

In any event, the CQB round mentioned in the OP was a 70 gr "Brown Tip". We almost certainly want to at least match, if not exceed, the sectional density of that bullet.

Why be concerned at all about sectional density? Isn't maximizing (or optimizing) terminal effects what should be important?

[JASmith]

Why would that be "more meaningful"??? In CQB, how often is there a barrier between shooter and target?

Are you planning on shooting naked people?

If a bare gel test is not relevant to those needs, why is it done in military and law enforcement tests?

The bare gel test is but one of several tests the FBI requires. IIRC the remainder all have a barrier of sort. A prepared adversary will have better barriers.

Why be concerned at all about sectional density? Isn't maximizing (or optimizing) terminal effects what should be important?

Terminal effects depend on penetration through barriers and flesh. Sectional density is a major factor in penetration depth.

Anticipating one of the likely follow-up questions: The product of sectional density and velocity (momentum density) is how the sectional density plays into terminal effects. It is easier to increase that product by increasing bullet weight than by increasing bullet velocity.

[stanc]

The bare gel test is but one of several tests the FBI requires. IIRC the remainder all have a barrier of sort. A prepared adversary will have better barriers.

Well, if you consider clothing as a significant barrier to rifle bullets, then yes, that "barrier" test should be done. I rather doubt that the performance of a TSX (no matter what weight or caliber) will be noticeably different than the same bullet against bare gel.

The only other barrier likely to be encountered in CQB (against the current enemy, for which the TSX "brown tip" load was created) is a loaded AK mag in a chest pouch. Body armor isn't likely to be worn.

Terminal effects depend on penetration through barriers and flesh. Sectional density is a major factor in penetration depth.

True, but just how much penetration depth do you think is necessary? I've seen nothing to indicate that an 85gr TSX wouldn't have more than enough. The wound cavity of the 120gr 6.5 TSX is substantially inferior to that of the 85gr 6.8 TSX, and the 22+ inches of penetration depth is far more than needed. Or are you planning to get two opponents with one shot?

I'll rephrase the last question in my previous post: Why do you "almost certainly want to at least match, if not exceed, the sectional density of that [70gr 5.56 TSX] bullet"? Is there something vital about that SD?

[JASmith]

Well, if you consider clothing as a significant barrier to rifle bullets, then yes, that "barrier" test should be done. I rather doubt that the performance of a TSX (no matter what weight or caliber) will be noticeably different than the same bullet against bare gel.

The only other barrier likely to be encountered in CQB (against the current enemy, for which the TSX "brown tip" load was created) is a loaded AK mag in a chest pouch. Body armor isn't likely to be worn.

True, but just how much penetration depth do you think is necessary? I've seen nothing to indicate that an 85gr TSX wouldn't have more than enough. The wound cavity of the 120gr 6.5 TSX is substantially inferior to that of the 85gr 6.8 TSX, and the 22+ inches of penetration depth is far more than needed. Or are you planning to get two opponents with one shot?

I'll rephrase the last question in my previous post: Why do you "almost certainly want to at least match, if not exceed, the sectional density of that [70gr 5.56 TSX] bullet"? Is there something vital about that SD?

There is a reason a heavy copper alloy bullet was selected for the CQB mission. The sectional density and ruggedness of this bullet compared to other choices available tell us that penetration is a significant issue.

Even today, CQB frequently involves impromptu barriers, vehicles, buildings, etc.

Advocates of the Grendel, or any other cartridge, must look beyond the current conflict. The one certain thing is that at least some of our adversaries will adopt body armor.

My understanding of the issues, based on a long association with the weapons R&D community, is that these seemingly unreasonable requirements will be imposed on new acquisitions.

Of course we know that every one of us, including me, will get the issues wrong on at least one point. The challenge is getting a read on what the requirements will be.

One could track the Commerce Business Daily for RFP announcements in this area. There is a possibility that a request for proposal will be released. If we are lucky, we could get an unclassified public release approved requirements document by responding.

[stanc]

There is a reason a heavy copper alloy bullet was selected for the CQB mission. The sectional density and ruggedness of this bullet compared to other choices available tell us that penetration is a significant issue.

No doubt SOCOM had a reason for selecting the 70gr TSX over lighter weights. But, even if you're correct in assuming that sectional density was a factor, it doesn't necessarily follow that the same SD would be required to get the optimum terminal effects with a TSX in a larger caliber.

Even today, CQB frequently involves impromptu barriers, vehicles, buildings, etc.

Granted, but my position is that there is likely to be no significant difference in barrier penetration capability between 6.5mm TSX bullets of various weights, while it seems very likely to be a clear superiority in wound trauma with lighter TSXs, due to their higher impact velocity.

Advocates of the Grendel, or any other cartridge, must look beyond the current conflict. The one certain thing is that at least some of our adversaries will adopt body armor.

I submit that's a non-issue. If it's soft armor, TSX bullets of any weight will penetrate it easily. And if it's hard armor, no 6.5mm TSX bullets will penetrate it, regardless of sectional density.

[JASmith]

I will concede that some opinions are dearly held.

A lot of the myths can be dispelled by a serious review of the penetration and wound mechanics literature accompanied by working closely with someone who understands that literature. After that, some of the issues raised can be resolved through testing.

Problem is, as Bill Alexander has pointed out, that setting up credible tests is an enormous challenge. For example, bare gelatin tests tell only part of the story even for large game in the hunting world.

One would need to get access to authoritative definitions of test targets to define the test regime.

[stanc]

A lot of the myths can be dispelled...

What myths??? Nobody has mentioned myths.

Problem is, as Bill Alexander has pointed out, that setting up credible tests is an enormous challenge.

What he actually said was that he objected to informal tests such as I proposed, and that he wants "well orchestrated" testing (presumably to make 6.5 Grendel look as good as possible).

Credible tests are not an "enormous challenge" at all. Even if one views some informal tests as "not credible," procedures used by the Army and FBI can be found online.