Weird .308 load question

G

Greg500sw

New member
I'm kind out of ideas so I figured I'd ask for help.

I loaded up some long range loads for my .308 and my chronograph has me in doubt.

My rifle has a stock Savage 12 bull barrel cut to 17 inches.

My load is a 178 g AMAX over 41.5 grains of H4895

The Hodgdon website has 42.7 as max for a 175 g bullet at 2,647 from a 24" tube.

My chronograph has an average of 2626 from my rifle with a load more than a grain less than max, so I'm kind of skeptical because no matter how I look at it, Short barrel plus less powder never equals same velocity.

I know it wasn't the loads causing a bad average, only 41 fps max spread with a standard deviation of 14.

Also, right before the .308 I ran some .223 through it that were right on the correct velocity.

So what do you fellas think? Should I trust the numbers, or not?
 
How far from the chronograph is the rifle? I've seen a number of folks struggle with errors from being too close to one. I'd use 15 feet to the centerpoint between screens. Your velocity should be in the 2400 fps range, though a very slow barrel can shoot low by a couple hundred fps or so, a fast barrel doesn't usually shoot high by that kind of number.
 
I'd say that your chrono should probably be double checked....

Record your real drop between 100 and 200 yards, although 200 and 400 yards would be better.

Then plug in the 178 Amax Litz data into JBM and drop the velocity down until you get to the velocity that matches your reality.

Jimro
 
I don't think you're off as much as you think. Shorter barrels don't lose as much speed as a lot of folks think. A starting load of 40 gr should give you close to 2500 fps. Even with the shorter barrel I'd expect speeds very near 2600 fps from your 17" barrel and a max load.

If you feel the chronograph is correct you may have made a mistake in loading and have more powder than you think. If the load is accurate and is showing no pressure signs it is probably OK.
 
It varies with expansion ratio. A good rule of thumb with .308 is 25 fps/in of barrel. So, as compared to a 24 inch standard SAAMI test barrel, you expect about 175 fps less. As the barrel gets shorter, the actual drop per inch increases as the muzzle pressure is increasing in the shorter barrel, and so an inch you lop off at the short end provides more bullet acceleration than an inch you lop off the end of a long barrel. By the time you get to a 32", you lose only about 15 fps/in.

For the above reason, QuickLOAD shows about 190 fps loss for a 17" barrel as compared to a 24" barrel shooting the starting load. The table below comes very close.

Barrel%20Length%20Velocity%20Approximation_zpsvmsocp0k.gif



Greg500sw,

Sounds like you eliminated muzzle blast causes. Other factors can give you false readings. A chronograph set on light colored ground can pick up light glints off the bullet, causing false triggering. This gets worse as the bullet gets bigger. Setting the unit on black plastic can help with that. Specific light conditions can cause problems. You'll note that at the IPSC nationals they put two chronographs in tandem inside a an opaque tunnel with infrared light sources operated by battery to get maximum consistency.

To see if the light conditions are affecting your unit, take a .22 LR rifle with you and a box of match grade ammunition. Match ammo fired from a .22 LR rifle barrel will be within 50 fps of the velocity claimed on the box, making it a good coarse check on the chronograph setup. This is for reasons of expansion ratio being so high for the cartridge in a rifle that it runs out of adequate pressure to overcome friction at around 16-20 inches, depending on the load, and the bullet actually starts to lose velocity slightly after that. The point is it makes full use of the energy in the powder and isn't subject to the variables of exact ignition timing and exact friction that much more affect a gun that's still got a lot of pressure when the bullet clears the muzzle, as centerfire rifles normally do.

Sometimes you can get a "fast barrel". One that has a tight bore that raises pressure, so you should be watching for pressure signs. Make sure your bore isn't building up copper fouling, which can raise pressure. But I have to say, I don't recall a fast barrel that made as much increase in velocity as your readings indicate.
 
Your velocity readings are what would be expected with the longer barrel. If the chrono readings consistent, would ask what brass, primer and COL you are using?

A tight bore or chamber, heavier brass, mag primer or bullet seated into the lands could be increasing your pressure/velocity. I would not be expecting 2600 fps from a 17in barrel, and shoot a fair amount out of 16-26 in barreled 308s.
 
zeke said:
Your velocity readings are what would be expected with the longer barrel. If the chrono readings consistent, would ask what brass, primer and COL you are using?



A tight bore or chamber, heavier brass, mag primer or bullet seated into the lands could be increasing your pressure/velocity. I would not be expecting 2600 fps from a 17in barrel, and shoot a fair amount out of 16-26 in barreled 308s.
Click to expand...



41.5 H4895
Once fired Winchester brass
Winchester large rifle magnum primers (all I currently have)
OAL 2.821

No stick in the chamber after firing, no pressure signs on brass, and the bullet isn't jammed into the lands.
 
Winchester is one of lightest 308 cases around, 41.5 grains H4895 is 1.3 grains above Hdy 9th edition max load with a standard Fed 210 primer/178 A-max bullet.

if your bullet has a some jump to the lands, would suggest reducing your load down at least 2.5 grains and work up to the velocity expected. While not exact, velocity is most practical/easiest way of judging pressure.

Cases sticking in chamber is WAY OVER PRESSURE, and while using case/primer indicators can demonstrate increases in pressure, they are unreliable ways to judge if you are approaching, at or above max pressure.

There ain't no free lunch. If your chrono is reliable, higher velocitys usually mean higher pressure.

Some manuals (older hodgen annual) show a slightly higher max with Sierra 175 hpbt and Win case, but with standard Win primer.
 
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Am not familiar with Speer's 180 sp, but the more hand loading data sources the better. Especially ones using the same bullet you are using. Still am of belief your load is over pressure, likely due to powder charge and mag primer. Fed 210M is a mild match primer.
 
I run 42 grains of H4895 behind a 168g BTHP and get awesome accuracy out of a Sig 3000. Below is a screenshot of the specs on my cartridge. Pay no attention to the "4896", I drug the cell down in excel and it auto generated the next digit. Also, I'm running a 24" barrel.



I also have a custom .308 throated for 208 AMAXs, with 45.5g of RL17 I'm getting 2535 fps out of a 26" barrel. Just something to use as reference.

*** This is raw data from my own Excel files***

c1c941d10987781a1a4e4fe52fe05d2b.jpg






Sent from my iPhone using Tapatalk
 
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Guys,

Please read the board rules on posting copyrighted materials.


zeke said:
if your bullet has a some jump to the lands, would suggest reducing your load down at least 2.5 grains and work up to the velocity expected. While not exact, velocity is most practical/easiest way of judging pressure.
Click to expand...

Until you get a bullet pretty deeply seated, jump to the lands usually reduces pressure rather than raises it.

Velocity won't let you judge peak pressure, only average pressure all the way down the barrel. If two different powders give you the same velocity in the same gun using the same brass and primer and bullet, all you know for sure is that the average pressure was about the same. This average includes the start pressure, the peak pressure and the muzzle pressure. If you get this a particular velocity with a fast powder, you'll note the charge weight is lighter than the amount of slow powder you need to reach the same velocity. The lighter fast charge makes less total gas, so its muzzle pressure is lower, and the only way it can then achieve matching velocity is if its peak pressure is higher. That is so it can makes up for acceleration missing near the muzzle with extra acceleration near the peak.

Even when you limit yourself to using the exact same components as the test data used, the peak pressure is only going to match if the expansion ratio of your rifle and the test rifle match. This and the above are why every set of chronograph instructions I've read say not to try to judge pressure with the instrument.
 
I posted my own data, from my own spreadsheet. Just trying to help the guy out.


Sent from my iPhone using Tapatalk
 
sorry, wasn't talking about two different powders, but about comparing velocity with a reloading manual, using same powder, components. If you are getting a lot higher velocity's than a manual states or than what is expected, the likely cause is higher pressure. If you are getting higher velocity with a much shorter barrel, the likely cause is higher pressure.

Not many I know have the equipment to measure pressure the whole way down the barrel, or pressure at all. Am of belief there is a reason they print the load charges and velocity's in manuals.
 
but about comparing velocity with a reloading manual, using same powder, components. If you are getting a lot higher velocity's than a manual states or than what is expected, the likely cause is higher pressure. If you are getting higher velocity with a much shorter barrel, the likely cause is higher pressure.
Click to expand...

Likely higher pressure, but not guaranteed.

One thing you cannot duplicate is the gun used in the testing of the reloading manual data. And the exact INDIVIDUAL gun used can make a difference, more than you might think.

I've seen guns firing the same ammo, with the same barrel lengths show 100fps difference in velocity. Its not common, but its not unheard of. Usually the difference is half that, or less, but it can be 100fps or even more, and still be perfectly safe, and while its uncommon, it is still normal.

Other people's reloading data are guidelines, not laws of nature. You (and I) will likely get something close to what they did, using their data and components. LIKELY, not guaranteed. We may get something close, we may get exactly what they got, or we may get something else entirely. GUIDELINES!!!

Every gun is an individual, and can have widely different results. MOST are pretty close to each other in this regard, but there ARE individual guns at each end of the bell curve, and this is normal. Yours may be "slower" than average, or it may be "faster". Don't sweat a handful of fps on paper.
 
You're right nothing is guaranteed, but there are minimal choices for the average handl oader to judge safe pressure, and every single situation is different. In this case the poster is using data from a different bullet and substituted a magnum primer. The charge of powder is above what the bullet manufactures max data is for the bullet he is using. Wisely he is using a chronograph. Within very differing guidelines, his velocity is well above (opinion) what can be expected from a heavy bullet in a short barrel.

It certainly is inexact, but perhaps preferable to assume it is a safe pressure if his primers don't flatten, and the case doesn't stick?

And measuring velocity isn't guaranteed either. Have used powders where the velocity actually decreased with increased charge weight lil gun in (pistols). Another very general statement is that sometimes the incremental velocity increases decrease with increased charges. Say that one fast.

I have several duplicates of firearms, and am very cognizant of the differing velocity's, even when they are the exact same model.
 
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