PMC - 147 gr 2550 fps?

[BeeKay]

I bought a chrono and it's a fairly low end one, because it was affordable.
So I'm suspecting there may be some error.

Loads that are listed as having a particular velocity are slower than expected, in the testing range between 2000 to 3000 FPS

So I fired some factory loads : PMC - 308 - Winchester - 147grain FMJ-BT, from my rifle with a 20" barrel and I got 2550 FPS avg.
This is about 200 FPS slower than what's listed on the box.

Which supports the idea that there may be an error with my chrono, because other loads I've tested have been about that much slower than I'd been expecting

Has anyone else chron'd those PMC loads to see if the velocity matches their advertised?

I'm actually leaning toward having to get into troubleshooting my chrono.
It's a Pro-Chrono Digital

Thanks in advance - Any advice appreciated

[ligonierbill]

The classic way to check your chronograph is to get a box of 22 LR match ammo and run that. Velocity for those rounds should be consistent with that on the box. It is not unusual for actual velocity to vary from advertised for commercial centerfire ammo. You have a barrel that is likely shorter than they tested, but I had two 30-06 rifles of the same barrel length that differed by more than 150 fps with my handloads.

[BeeKay]

I'm seeing in YouTube videos that guys have tested the same stuff that I mentioned and got velocity that matched what's advertised on the box.

I'm looking at what's mentioned in the brochure for the chrono and it seems like I did it right.

[Shadow9mm]

Your chronograph is fine. Most rifle ammunition is tested in 24-26in barrels at the factory. I have even seen where some were tested in 28in barrels. You have a 20in barrel, so it is going to be slower. If you look at loading data, Hodgdon's site for example, and look up 308, you will see their test data is from 24in barrels. There is no industry standard barrel length for testing and each gun is an individual.

Also, temperature effects things. Some powders are more temperature stable than others. But you can get big velocity swings with some powders.

[zeke]

PMC appears to have tested it from a 24 in barrel, and a 20 in barrel could easily lose 100-130 fps?

https://pmcammo.com/product/bronze-308b/

But you still appear to be below what could be expected. It might help to know what rifle you are using and at what temp? Know anyone else with a chrono to check yours?

edited to show inadvertent partial repeat

[Mal H]

Originally Posted by Shadow9mm

Most rifle ammunition is tested in 24-26in barrels at the factory.

I'm fairly certain that is the key to the difference you are seeing. You're not only in the ballpark with the velocity you recorded, but I would say you are in the infield for that particular round and barrel length.

[And, since this isn't a reloading issue, but more of a shooting gear issue, I'll move it to the Gear and Accessories forum.]

[BeeKay]

It's a Browning Lever Action with a 20" barrel and the temp was about 50 farenheit when I tested.

Probably the best idea is to check it against someone else's chrono.
I'll have to make an effort to locate someone who has a chrono that's known to be accurate, or hope to meet somebody by chance at the range.

Thanks for the suggestions.

[Shadow9mm]

50f is probably on the cooler side, between the barrel length and temps, I would say your chrono was most likely reading correctly. Don't over think it.

If you do insist on over thinking it, here goes

This chronograph, along with most others, is only accurate to within +/- 1% of the velocity shown. If you account for 1% over and 1% under or 2% total, at 2550fps that give you a possible error of up to 51fps.

If you account for barrel length at 100-150fps of the variation, temperature at a possible 10-25fps difference, and the accuracy of the unit at +/- 1% of the listed velocity for 51fps this puts you right where you are and fully accounts for the difference listed on the box vs your rifle.

Having used a chronograph for a while. I trust it a lot more than what is on the box. I generally look at the numbers on the box as a rough estimate. Then I shoot them over my chrono to see what they actually do in my gun and go from there. Generally in my experience, all ammo will be slower than listed on the box unless you have a real long barrel, and it is a hot day.

[44 AMP]

one additional factor to consider besides your barrel length is the barrel itself.

Different guns, even with the same barrel length can, and do shoot the same ammo at different speeds. Some guns are faster and some slower. Most are pretty close, but both ends of the bell curve are out there and you might just be at the slow end. Machts Nichts

The difference could be 100fps or even more. Usually its less but 100fps isn't unheard of.

I've personally experienced this. It's not fantasy, it exists. If you happen to have a "slow" barrel (and the only way to know is chronographing) AND its also a short barrel, that's a double whammy and you aren't going to get the same speed testing got with a different barrel especially a longer one.

I've seen the same .357 ammo clock 3 different speeds from 3 different guns, all with (nominal) 6" barrels. Gun A = 1620fps Gun B = 1670fps and Gun C -1720fps. All shot the same afternoon, over the same Chrony, with ammo from the same box. It can happen, and there's nothing you or anyone else can do about it, and it doesn't mean your chronograph is bad or out of calibration.

[jmr40]

MC appears to have tested it from a 24 in barrel, and a 20 in barrel could easily lose 100-130 fps?

In my experience you don't lose anywhere near 100 fps from a 308 due to 4" less barrel. You run into differences in the tolerances of individual barrels and chambers that have more to do with velocity differences than a few inches of barrel makes.

Even from the same barrel length you can see 100 fps or more difference in velocity. I own several 308 rifles with 18", 20", and 22" barrels. The one with the 20" barrel shoots the same loads faster than any of the ones with 22" barrels. And there is 90 fps difference between the fastest 22" barrel and slowest 22" barrel. The gun with the 18" barrel is slower, but almost equals one of the guns with a 22" barrel.

If you start with a barrel with a known velocity and cut it shorter expect to lose 10-15 fps for each 1" you cut it shorter with 308. Magnum cartridges holding much more powder would see slightly more speed loss/inch. But when you're comparing different rifles, even with the same barrel length almost anything can happen. I've seen 130 fps difference between two different 30-06 rifles, both with 22" barrels.

But 2550 fps doesn't seem out of line. The numbers published by ammo manufacturers are often optimistic. And with budget brands more optimistic than normal. You'd probably see 50-60 fps more speed with a 24" barrel and you could just have a "slow" barrel. With all of those factors combined 2550 isn't terribly out of line.

[bamaranger]

My chrono makes a liar out of me all the time. A load that I think will make "X" velocity will yield "y". I test fire my chrono with a BB gun! Given the variations in chambers, barrel length and polish/finish and ammo......2550 for a .30/157 does not seem all that unreasonable.

[Bart B.]

A given load will shoot slower in same length barrels with larger inside groove diameters compared to smaller ones. Slug your barrel then compare its groove diameter to that of the bullet.

A few percent spread in charge weights across all lots of powder is needed to have the same velocity. Commercial canistered lots have smaller spreads than bulk lots arsenals use in military ammo.

The harder the rifle's held by humans, the faster bullets will leave. I've seen several dozen fps spread across several people shooting the same stuff from a bench top.

SAAMI says cartridges shooting bullets 2500 to 3000 fps have about 25 fps change per inch of barrel length.

All ammo makers don't use SAAMI spec test barrels and chronographs. Often is a typical firearm chambered for the cartridge

Check this out..... http://bulletin.accurateshooter.com/...barrel-length/

Best accuracy happens when the bullet diameter is at least .0004" larger than the barrel's groove diameter. Some 30 caliber match barrels have .3065" groove diameters so 7.62 NATO M80 ball ammo with. 3070" bullet diameters will have good accuracy in competition at long ranges. Their 30 inch length makes 147 grain bullets leave about 3000 fps so they'll stay supersonic at 1000 yards.

[BeeKay]

OK - Thanks for all the helpful information

[Bart B.]

Originally Posted by Shadow9mm

There is no industry standard barrel length for testing and each gun is an individual.

SAAMI has industry standards for all test barrel dimensions. 308 Winchester data is on page 306 in the rifle book:

https://saami.org/technical-informat...mber-drawings/

Everyone doesn't follow them. They don't have to.

[T. O'Heir]

"...when I tested..." Chronographs can be affected by the light conditions too. The barrel length and its rifling twist matter too.
"...SAAMI has industry standards..." SAAMI is who sets the Standard for North American ammo. Dunno if PMC, who are Korean, follow SAAMI or not. And, like you say, SAAMI does not decided what barrel length is used for testing. Hodgdon, for example, tested their .357 loads with a 10" barrel.
"...lose anywhere near 100 fps..." One of the gun rag writers, years ago, tested that by cutting off an inch at a time. He, forget who, concluded you lose about 100 FPS per inch.
"...YouTube videos..." Aren't exactly science.

[Bart B.]

Originally Posted by T. O'Heir

"...when I tested..." Chronographs can be affected by the light conditions too. The barrel length and its rifling twist matter too.

Any data available for rifling twist variables?

I don't think there's more than about a 1 degree spread across all twist angles normally used for a given cartridge.

[ballardw]

Originally Posted by Bart B.

Any data available for rifling twist variables?

I don't think there's more than about a 1 degree spread across all twist angles normally used for a given cartridge.

One experiment here:
http://bulletin.accurateshooter.com/...le-velocities/

For the given .30 caliber 175gr Berger OTM bullet loaded to velocities around 2660 FPS the conclusion was a loss of about 1.33 FPS per inch of twist.
They were also using different types of rifling, which hasn't come up in this discussion yet.

I'm pretty sure YMMV.

[Bart B.]

Were all barrel's internal dimensions the same same?

[ballardw]

From the article:

We acquired six (6) barrels from the same manufacturer (Bartlein), all the same length and contour, and all chambered with the same reamer (SAAMI spec .308 Winchester). All these barrels were fitted to the same Savage Precision Target action, and fired from the same stock, and bench set-up. Common ammo was fired from all six barrels having different twist rates and rifling configurations.

That's what was provided. One might assume given the same maker the internal dimensions started the same but as I said, different types of rifling were also used. So I have no idea what impact that might have.