7.62x51 battle rifle

[Younggun]

That's a fair statement, as it was just a quick calculation and I can't personally attest to accuracy of the numbers.

That said, I do believe those numbers are pretty reasonable, assuming the source data was good. Here's how I got those numbers:

I didn't have any chrony data on the 13.5 directly, despite having a very similar build a while back (was actually my first AR platform self-build, one of the few I regret having to sell; old pic at bottom of post).

However, I was able to find some data published in the following TTAG article, and used the published speed of 2380fps for the 168gr projectile:

https://www.thetruthaboutguns.com/2...t-barrel-length-muzzle-velocity-and-accuracy/

(graph from article - mods feel free to remove if necessary):
View attachment 151763

I then just used the 2380fps MV in my BC and ran the tables with atmosphere...pretty straightforward, and a good starting point short of rifle/load specific data...

View attachment 151764

View attachment 151765

View attachment 151766

Didn't include links here to studies on bbl length harmonics as figured would be too much for one post...

And pic of my old similar build (this one had 13.7 bbl with pinned comp = 16.25" oal):
View attachment 151767

I’ve read the articles on harmonics and find them pretty interesting. I feel that info is accurate.


I’m curious what you used for the ballistic coefficient and whether it was G1 or G7.

Running Sierras G1 variable BC and 2400fps velocity and my current altitude with a generic atmosphere I get subsonic at 813 yards.

Using the G7 BC from StrelokPro I get closer to 750 yards.

That said, the real killer is the same thing that plagues all .308 at longer ranges. For a standard .308 running 2700-2800fps MV you have to deal with the 800 yard “wall”. When the bullet seems to be hit by a sudden gust of gravity and your elevation must be dialed in nearly perfect. Starting out 300fps slower you will hit that wall much faster. I show a 50” drop from 750-800 yards using the Sierra variable BC data.

I used “Ballistic” for the calculation because it’s easier to play in than StrelokPro. Strelok does a better job of predicting when a bullet will enter the transonic range though IMO and usually says it happens a littler earlier.

And I’m not bashing .308 in any way. I stuck with it for a long time while guys I shot with got on the Creedmoor bandwagon. Ran my 18” barreled Savage to 1k using 175gr SMKs but never got further. It seemed to be a shotgun pattern at 1,200.


Sent from my iPhone using Tapatalk

 

[CyberWolf]

I’m curious what you used for the ballistic coefficient and whether it was G1 or G7.

Running Sierras G1 variable BC and 2400fps velocity and my current altitude with a generic atmosphere I get subsonic at 813 yards.

Using the G7 BC from StrelokPro I get closer to 750 yards....

I just used the built-in Litz BC data (.218 BC) with a G7 drag model...

That said, the real killer is the same thing that plagues all .308 at longer ranges. For a standard .308 running 2700-2800fps MV you have to deal with the 800 yard “wall”. When the bullet seems to be hit by a sudden gust of gravity and your elevation must be dialed in nearly perfect. Starting out 300fps slower you will hit that wall much faster. I show a 50” drop from 750-800 yards using the Sierra variable BC data.

What you're describing (the "wall") is the transonic shockwave/destabilization which occurs as the projectile approches the speed of sound during deceleration, which (SofS) varies based on temp, barometric pressure, and humidity (why using density altitude for normalization is much easier, and a better way to measure atmospheric resistance).

I would wager that I'm at a higher DA than you (e.g. thinner atmosphere/less resistance), which is why you're seeing a shorter range prior to the round going transonic....

ETA: G7 is likely going to be the preferred drag model for that projectile

 

[Younggun]

I just used the built-in Litz BC data (.218 BC) with a G7 drag model...

View attachment 151777






What you're describing (the "wall") is the transonic shockwave/destabilization which occurs as the projectile approches the speed of sound during deceleration, which (SofS) varies based on temp, barometric pressure, and humidity (why using density altitude for normalization is much easier, and a better way to measure atmospheric resistance).

I would wager that I'm at a higher DA than you (e.g. thinner atmosphere/less resistance), which is why you're seeing a shorter range prior to the round going transonic....

ETA: G7 is likely going to be the preferred drag model for that projectile

The wall I’m describing isn’t the transonic barrier. It is the fact that the bullets is moving relatively slow and dropping very quickly in relation to the horizontal distance traveled.

Turbulence as the bullet transitions from supersonic to subsonic speed is why U had a shotgun pattern at 1200 yards.


And elevation is the probable cause for our differences. I used 642 feet above see level because that’s where I live with a temp of 65* and relative pressure of 29.92. When shooting I use Density altitude instead. Would have been better for this discussion but I was being lazy and did’t pull it up.

That said, the limiting factor for .308 in the field will often come down to the poor trajectory at long range. Not a big problem at the gun range if you have good DOPE and you know the exact distance to the target. A bigger issue when your ha one to estimate the range and the bullet is dropping in like a mortar shell.


Sent from my iPhone using Tapatalk

 

[Kosh75287]

TheMailMan said: The biggest problem with both is they are hell on brass.

That’s true, I’d never try to reload anything that was shot out of my HK-91 or PTR, but I wouldn’t hesitate to pull either one out when I needed an MBR.

Once-fired brass that's been shot out of a G3-pattern rifle CAN be reloaded for BOLT ACTION rifles (at least once, and usually 2 or 3 times). It's not IDEAL, but it can work, as long as one doesn't expect miraculous performance. The key is to use a small-base die to get it back to spec, then just neck-size it from there. The neck flutes never go away before the brass shows signs of failure, but the reloads I've used with such brass worked quite well for duplicating M80 Ball (AGAIN, in a bolt-action rifle), or a reasonable approximation of factory 150gr. SP (usually 2 or 3 times).

As for a .308 Cal MBR, keep the 1979 H&K 91. If you want an MBR that is kinder to brass, I'D suggest a Springfield Armory Supermatch or National Match. A well-built STG-58 (FAL pattern, with better tolerances) might also work for you, depending on what you will settle for in accuracy. I have no experience with the AR-.308s, so someone else will have to chime in on those.

 

[CyberWolf]

And I’m not bashing .308 in any way. I stuck with it for a long time while guys I shot with got on the Creedmoor bandwagon. Ran my 18” barreled Savage to 1k using 175gr SMKs but never got further. It seemed to be a shotgun pattern at 1,200.

One thing I forgot to add in last post, the idea for me (in this case) wasn't to optimize for ELR/PRS, but as a general purpose rifle which can be quick/manuverable for CQB/barrier-penetration, while retaining ability for quick intermediate-to-long range hits, and taking advantage of the ubiquity of .308W/7.62N ammo...

 

[CyberWolf]

The wall I’m describing isn’t the transonic barrier. It is the fact that the bullets is moving relatively slow and dropping very quickly in relation to the horizontal distance traveled.

Turbulence as the bullet transitions from supersonic to subsonic speed is why U had a shotgun pattern at 1200 yards.


And elevation is the probable cause for our differences. I used 642 feet above see level because that’s where I live with a temp of 65* and relative pressure of 29.92. When shooting I use Density altitude instead. Would have been better for this discussion but I was being lazy and did’t pull it up.

That said, the limiting factor for .308 in the field will often come down to the poor trajectory at long range. Not a big problem at the gun range if you have good DOPE and you know the exact distance to the target. A bigger issue when your ha one to estimate the range and the bullet is dropping in like a mortar shell.


Sent from my iPhone using Tapatalk

I believe we're talking about the same thing, I was jusy using DA as shorthand for my elevation, etc.

Also, agree that there will be one hell of a drop at that distance, but it is predictable prior to going transonic (assuming proper wind calls).

I use DA to simplify the DOPE calculations and keep printed tables for my standard range of DA's, but am typically lazy as well and just grab current atmospherics for the calculator (especially if trying to squeeze out the most accurate numbers possible).

Same thing on range, pretty much stick to ffp/mrad hashed/tree reticles (with a couple exceptions) and can do the range calculations manually if necessary & markup a range card, but am often lazy with this and just use a Sig Kilo2k...

 

[Younggun]

One thing I forgot to add in last post, the idea for me (in this case) wasn't to optimize for ELR/PRS, but as a general purpose rifle which can be quick/manuverable for CQB/barrier-penetration, while retaining ability for quick intermediate-to-long range hits, and taking advantage of the ubiquity of .308W/7.62N ammo...

It’s all good. I like the .308 round and if I build or buy a large frame semi auto it will be in .308 instead of 6.5.

Like you, I would build for the intended use. My use would be slightly different though primarily for 400-800 yard targets and fast acquisition (which is why I would want to avoid the steep ballistic curve past 800 yards).

Admittedly, a 13.5” barrel would be better at serving the dual purpose you describe than an 18-20” barrel. But I wouldn’t have the CQB role in mind either.


Enjoyed the discussion.


Sent from my iPhone using Tapatalk

 

[Eli]

Alright,

I've got a HK 91 that I purchased back in '79. I haven't shot it in well over 20 years. I've been thinking of parting ways with it but really want to keep a 7.62x51 MBR in the safe.

Give me your suggestions on a good one. Not interested in a FN-FAL.

If I'd seen this a few weeks ago, I'd say get a .308 Field Grade Special from the CMP - they were $850 delivered - and a used one might suit you yet.
If that HK is in nice shape, it's worth thousands, you could buy a PTR-91 GI, the .308 M1, and a pile of accessories and ammo for both and still have money left over.
As for myself, my 'main rifle' - my only rifle, really - is a BM59SL a/k/a BM59E:

If you want one, Shuff's Parkerizing is the only way to go; none of the cast receivers are in-spec and many reworked GI receivers are as well.

Eli

 

[baboon]

I asked my dad for an HK 91 when I was 16, in the 1970s. He ordered it at a local shop but all they did was make excuses. The money that was going to get it for me eventually went to buy me my first Wichita. Not as good an investment but a much nicer gun.

In those early years not a lot of them where being imported into the USA. I waited a year for my HK Coded dated(1979) H&K91. Still have it & have thought about selling it, but most likely won't until very hard times.

 

[V-Tach]

Alright,

I've got a HK 91 that I purchased back in '79. I haven't shot it in well over 20 years. I've been thinking of parting ways with it but really want to keep a 7.62x51 MBR in the safe.

Give me your suggestions on a good one. Not interested in a FN-FAL.

Why no FAL interest? Just curious.....

 

[Darqhelmet]

Quality AR10 pattern or roller locker.

 

[TheMailMan]

Why no FAL interest? Just curious.....

I just don't want one.

Nor do I need a 800 yard rifle. Hell, the HK isn't a 800 yard rifle.

I know the brass can be reloaded but it really does cut down the brass life.

I'm not even sure I actually need a 7.62x51 MBR.

 

[V-Tach]

Honest answer.....I respect that.......

 

[satx78247]

To All,

Do any of our members KNOW a person who is competent to assemble a G3 parts kit on an American made receiver?

IF you know of such a person, who does this task at a reasonable price, please PM me with contact info.

yours, satx

 

[oldag]

I'm not even sure I actually need a 7.62x51 MBR.

Of course you do.

 

[SC-Texas]

POF Revolution

Sent from my SM-N920V using Tapatalk

 

[CyberWolf]

POF Revolution View attachment 152133

Sent from my SM-N920V using Tapatalk

Funny you should mention that...

Have been looking real hard at the 12.5 Revolution w/ pistol brace (would likely add a Law folder as well) last few days, only reason I haven't ordered one yet is the Burnt-bronze model appears to be sold out everywhere....

 

[McCrapper]

SCAR 17, it is so much better than the HK. Expensive, but worth it.
If you want to stay with HK, then how about a MR762.
Oh, and I love HK91’s. I bought a HK91A3 in 1983, and loved it.

Worth every dime. The best choice of the bunch.

 

[TheDan]

The biggest problem with both is they are hell on brass.

I feed my PTR steel cased crap and don't worry about it. They definitely aren't ammo sensitive.

 

[TheMailMan]

I feed my PTR steel cased crap and don't worry about it. They definitely aren't ammo sensitive.

But that would mean I have to BUY ammo!