Velocity shock, WTK...


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stubbicatt
December 25, 2008, 07:34 AM
I've posted this over at HKpro.com and received no input. I figured you the cognoscenti will have insight.

Guys.

I am trying to duplicate 7.62 NATO loadings with the Hornady 150 gr. FMJ bullets.

So yesterday I took my PTR91 to the range, set up the chronograph maybe 3 or 4 feet in front of the muzzle, which is equipped with a muzzle brake. No errors and readings pretty consistent. Would have put it out further, but the distance seems OK.

So I go online to find out what milsurp velocities should be, and the almighty internet gave a range, but out of the G3 barrel it is supposed to be 800 meters per second, or 2625 fps. See Wiki (http://en.wikipedia.org/wiki/Heckler_&_Koch_G3). (I verified the math on the conversion to fps. here. (http://www.calculateme.com/Speed/MetersperSecond/ToFeetperSecond.htm))

The first five rounds out the 18" tube were South African surplus, and averaged 2792 fps. Notably faster than the Wiki article suggests. OK so far I guess. SD was 35.

Next five rounds were handloads at 2841 fps average. SD 19
Next five rounds, +1 grain of powder over the first five averaged 2898 fps. SD 28.6
Last five rounds, +1 more grain, averaged 2968 fps. SD 24.

No pressure signs on the brass, and all loads below maximum by published data of the powder manufacturer, and data published by the bullet manufacturer.

What gives?

The 18 inch barrel should be a limiting factor on these loads. I would have expected lower velocities than out of a 20 inch barrel, or longer.

1) I think either the chronograph is wrong, or
2) The PTR barrel gets more velocity than a G3?, or
3) The South African Surplus is higher velocity than NATO spec., or
4) I don't know what...

Not sure how to interpret this data, and look for suggestions.
Anybody?

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gvnwst
December 25, 2008, 10:21 AM
4) I don't know what...


Wikipedia is wrong? They often are, on the more precise things. Search for actual data on nato surplus, it should give you a better idea on what the number actually is.

dakotasin
December 25, 2008, 10:32 AM
i don't have any idea what nato ammo is supposed to chrony at, but...

the 308 is an efficient case. many times you will find that shortening a barrel of an efficient case will have minimal impact to velocity numbers. i.e., lopping off 4 or 5 inches from a 308's barrel will have a far less impact on velocity numbers than something like a 300 win mag.

there are 'fast' barrels, and there are 'slow' barrels. it is possible you have a 'fast' barrel.

the chrony could be off, but as long as it is consistent and predictable, it won't have too much of an effect on your finished product. shoot a known variable over it to see if it is reliable.

MTMilitiaman
December 25, 2008, 11:15 AM
The most official velocity standard I could find with a quick Google search comes from the National Institute of Justice for their body armor testing. It shows the 7.62x51 M80 ball round @ 2700 to 2800 fps. No barrel length is given.

http://www.griffinarmor.com/ballisticsnij.html

As mentioned, barrel quality can affect velocities, sometimes even more than barrel length. A good quality barrel made to tighter tolerances can exhibit higher velocities than a lesser quality barrel. It's possible and even probable that JLD is putting a pretty good quality barrel on their rifles now days.

Over all, however, I don't see anything really noteworthy. The velocities you listed are right in where they should be. Maybe a little high, but not much. Nothing wrong with your chrony.

Art Eatman
December 25, 2008, 11:52 AM
I've not done any testing with a chrony, but by and large over many decades of reading commentaries, 150-grain bullets out of a .308 seem to pretty much be in the 2,700 to 2,900 ft/sec range.

I guess if I were ranking comparative performance, it would be handloads as the fastest, then factory, and then military as the slowest...

rangerruck
December 25, 2008, 12:16 PM
wiki is wrong; that speed sounds like the speed you may find for the match 175's, but not a 150. there are def countries that loaded to all diff speeds, i allways liked the s. african, very consistent. A bit more oomph than any nato round, I have found. I see no probs firing a 150 at 3000 fps, a freakin 17hmr does 2700 fps now, and produces more pressure than a 308, so i don't see any probs handloading to 3000. Maybe you will get less reloading from each piece of brass, but that would be about it. I also wouldn't recommend 3000 fps from a old milsurp rifle, but new made rifles, especially CZ, shouldn't see any probs at all.

Lone_Gunman
December 25, 2008, 01:10 PM
My guess is that your barrel is a little faster than most. Different barrels can make 100 fps difference.

browningguy
December 25, 2008, 05:37 PM
My guess is you need to move your chrony out to at least 10 feet where it belongs.

At 3-4' the gas exhausting may/will effect the readings, whether you want to believe it or not. And all other chonograph readings you see will be at 10-20' from the muzzle.

briansmithwins
December 25, 2008, 07:00 PM
PTR91 with a polygonal barrel? I seem to recall that one of the benefits of the polygonal barrels was that they had higher velocities.

BSW

stubbicatt
December 26, 2008, 08:52 AM
I may have to try the 10' distance for the Chrony.

Thanks guys for the input. I had read that the M14 with a 20" barrel was supposed to be 2750 fps.

Followup question: If my goal is to duplicate milsurp velocities, should I go ahead on and reduce the powder charge a scosh to get the averages down by 50 fps?

I think what I will do is to drop a grain of powder, and build loads at .3 grains increments and shoot for accuracy, and see if anything definitive emerges.

The barrel is not poly, it is 8 lands and grooves, very narrow, very shallow, lands.

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