Optimal 0-600 yds Barrel

[TA03827]

Hey everyone,

Searching for the optimal barrel length to fire 0-600 yds suppressed with enough terminal velocity at 600 yds.

This is for a combat rifle.

[grayfox]

What terminal velocity do you consider "enough"...
better yet, what KE or bullet performance do you consider enough.
For a particular class of game animal or *htf... rifle. or target...

What wt and type of bullet will you use? since "combat" will this mean only fmj? or is any pill ok.

[biodsl]

What terminal velocity do you consider "enough"...
better yet, what KE or bullet performance do you consider enough.
For a particular class of game animal or *htf... rifle. or target...

What wt and type of bullet will you use? since "combat" will this mean only fmThj? or is any pill ok.

These are all good questions. I'd add; what elevation is your area of operations? At sea level? Add barrel length.

[TA03827]

grayfox: At the very least 1320 fps at 600 yds - preferably, however, at least 1500. Only FMJ, anywhere from 100-123 gr.

biodsl: Elevations min at -300' max at 15,000'. I am aware this is a wild fluctuation. What are your recommendations for differing altitudes?

[biodsl]

My point is simply if you're never operating at less than several thousand foot of elevation you can get way with less velocity than what's needed at sea level.

Also, ballistics change a bit if it's 29 degrees vs. 79 degrees.

If you're willing to tolerate a 1,300 fps speed at 600 yards, you don't need anything more than a 16" barrel, even at sea level in below freezing temperatures. If you need 1,500 in the same situation (sea level at 29 degrees) you need a 20" barrel.

You can play with all this data at JBM Ballistics. Pick your preferred bullet from the drop down menu and then start playing with speeds and atmospheric conditions. http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

[grayfox]

Anything that works for 300' will also suffice for you at 15,000. Since a cartridge generates its own O2 for deflagration, 15,000 should still work although I have not shot at those heights.

So, Hornady has a 6.5mm fmj, 123 grs, BC 0.420 (G1)... even currently available!
I didn't see any other fmj's on ammoseek currently, so will use this one for examples.

Running some ballistics numbers, at 300 msl, this 123 gr fmj starting at MV of 2375 would drop to 1385 fps and 524 fp-KE at 600 yards. My BC calculator goes up to elev 10,000 and for that, 600 yds increases from 1385 fps to 1805 fps/ 890 ke.

Looking upwards to your 1500/600 yds desires:
At MV of xxx, 600 yds velocity of yyy:
MV 2550 fps, 600 yds 1503
2500 fps, 1469
2450, 1435
2400, 1401


So. Now I have some real world MV data from my 16" barrel, using a few 120 and 123 loads, magnetospeed (elev 324):

Factory Hdy 123 sst: 2432, sd-13
Factory Hdy-black (this was an older lot, might not be this for newer lots): 2432, sd-12
Factory Federal 120 fusion 120: 2429
Hand load, AR comp and cii450 primer, 2442.

The shortest civilian barrel being 16", it would appear that properly loaded 123 fmj's loaded similarly to these would meet or slightly exceed your minimum requirements. I have not shot any 14.5" barrels, and haven't as much data for my (once-upon-a-time) 12.7" barrel, but 16 certainly is a recommendation. Longer obviously if you like. More barrel, more MV.

To get to the 2475-2500 MV level you would probably need 20" barrel, of course note also that most of the faster Grendel powders are ball-type so fairly temp-sensitive (thinking hi-altitude here). AR comp and 8208 xbr are the more insensitive ones typically used.
The 2450-2475 starting MV is achievable from an 18" barrel.
2400-2450, as mentioned, from a 16".

Others more experienced than I might also weigh in.
This should be some of what you're asking about...

[grayfox]

One final thought: optimal? This is in the eye of the beholder. If you can live with 2400-ish at 300 msl, then 16" is easiest to haul around at all elev's, plus would be easier to handle if you ever want to use a suppressor.

[TA03827]

biodsl: Thank you for the link.

grayfox: Minus 300' -- below sea level. Will this lead to the further issues as biodsl suggested? Also, thank you in kind for the numbers.

[grayfox]

-300 elev, hmmm. Interesting.

Actually not totally sure, the only -300 places I know of, are very dry desert and hot. So really dry air, maybe slight density increase, also pretty hot most of the time? So less dense...
But then again, maybe below sea level at above -300 but maybe humid (tropical), however humid air is less dense than dry air... so not sure how all of those would play out. Sorry for the digression.

But...
I had never considered BC effects below sea level, but lo and behold my little calc will go down to -100.
So this is mostly an estimate using extrapolation. Also, note that humidity is not a part of this particular calculator.
Using the 2375, recall the 600 yd, +300 msl was 1385 fps.
At 0 msl, it calcs 1377 fps.
At -100, it shows 1374 (ke = 516).

I don't have this graphed out but it should stay above your min of 1320, probably around 1365+/-... more so especially if you look at starting MV's around 2400, which gives you a bit more wiggle room.

[Klem]

Tao,

That is interesting.

The deepest place in North America is Death Valley -280ft.

Where do you shoot?

[grayfox]

You might want to also consider putting in a little "margin" to accommodate fluctuations and other unknowns, that always seem to crop up. For instance, one load batch shooting a little slower or faster than the nominal or design MV. Newer barrels that could tend to be a bit "slower" than the nominal barrel. There's always the MV loss for lower than expected temperatures.

To mitigate these you might want to say 17-18" would be the min you would look at. I say 17 b/c lilja I believe makes a 17" barrel, plus a gunsmith or whoever you get the barrels from, might be able to deliver that. Or 18" is a more standard offering.
So, if you want a little margin, use the 18" as your min.

[TA03827]

grayfox, thank you again for the number crunching.

In terms of environmental effects, we will just have to play around with the grs a bit to see how things work.

Thank you for the barrel advice.

[StoneHendge]

Tao,

That is interesting.

The deepest place in North America is Death Valley -280ft.

Where do you shoot?

And underground combat rifle with terminal velocity of to 600 yards. We live in strange days indeed!

[grayfox]

The relatively new 6mm ARC cartridge, developed for a US spl ops group, uses match bullets in its round, and is a cousin of the grendel, also in AR15 frame. Its 18-20" barrels and 105-108 bullets, with the higher BC's, show more like 1750+/- fps-730ke at 600 yds and 500 msl. 1500 ft/s-547 ke at around 800 yds,
Might also think about that for your use, 18-20" barrels usually. Seems to have been developed for certain mission-specific needs of a marksman/sniper military type.
Not to talk you out of anything, just another option.

[myrifle]

Hey everyone,

Searching for the optimal barrel length to fire 0-600 yds suppressed with enough terminal velocity at 600 yds.

This is for a combat rifle.

First we need too know what action you define as a combat rifle?

[LR1955]

First we need too know what action you define as a combat rifle?

MR:

I think he ought to go into more detail than just the action.

He can also state overall weight unloaded and loaded, maximum length, trigger, sighting system.

Then he can get into mean rounds between failures, hit probabilities on an upper torso size target. I may have forgotten a couple of factors too.

Not sure how many factory loaded FMJ cartridges are out there for a Grendel aside from the Wolf stuff. Are there any?

LR55

[grayfox]

Other than Wolf, none that I know of. And those don't seem to be real accurate.
There are some Hornady 123 fmj bullets, so somebody could make some.
If an OTM or match bullet would do then there are a few mfrs that do that.

[TA03827]

grayfox, to the best of my knowledge, the current manufacturers of FMJ are Wolf and Hornady. As you've stated, Wolf is not a great option. Soft tips will not suffice. My knowledge on 6mm ARC is somewhat limited, but I will certainly look more into it. I was unaware of the velocities you've mentioned - it will definitely require another look.

[Klem]

grayfox, to the best of my knowledge, the current manufacturers of FMJ are Wolf and Hornady. As you've stated, Wolf is not a great option. Soft tips will not suffice. My knowledge on 6mm ARC is somewhat limited, but I will certainly look more into it. I was unaware of the velocities you've mentioned - it will definitely require another look.

If soft tips are an issue for your combat rifle are you concerned about the 1899 Hague Declaration? Is your country a signatory to the ICC?

If so, the Rome Statute which gave birth to the ICC ratified the Swiss Proposal permitting expanding tips for 'counter terrorism' in 1980. Australia, for example back in the 80's issued .223 Nosler 40 and 55gn ballistic tips (the black Lubalox-coated 'Combined Technology' rounds) to their special forces for counter terrorism. In 7.62mm it was 110 and 130gn Winchester soft points. The compelling argument being you need rounds that mitigate exiting the bad guy and continuing on to hit a potential innocent. Their wider conventional Army however still uses non-expanding rounds for a more conventional war (F1 and F1A1 equivalent to the SS109 STANAG/M855).

The US has always been concerned about the limiting nature of the Hague Declaration and is not a signatory to the Rome Convention (does not recognize the ICC). The M262 open tip match is like a compromise stance where the open tip is tiny, but it is still an open tipped bullet if you stare hard enough at it. Truth be known. militaries nowadays are more concerned about innovations in body armor, meaning expanding rounds requiring soft or open tips are superior on soft skins but just don't make it through armor. And of course there is no 6.5bullet that we know with a hardened steel or tungsten penetrator readily available on the market.

If only FMJ's fit the bill it would not be hard to negotiate with a small ammunition manufacturer to assemble batches of quality FMJ's to your specs. You could even source the components yourself and hand them over with a load in mind.

Where do you shoot?

[biodsl]

Lapua makes a 100 grain FMJ with a stunning BC of .250! Also check the BC on their 144gr FMJBT.

https://www.lapua.com/bullets/fmj/