308 Velocity Variation in 20" Barrel

[Numn]

Two TRG22's: First is a fixed stock, 26" barrel, Near brake and about 2000 rounds. Second is a folder with 20" barrel, no brake and about 150 rounds.

Load consists of new Lapua brass with no prep other than mouth chamfer in a Giraud trimmer.
175 SMK seated with Redding comp, don't currently have notes for ogive measurement but OAL was 2.800 to 2.810 depending on bullet tip.
44.0 gr Varget (Chargmaster)
GM210M Primer

100 yd accuracy is very similar for both rifles with the most significant variation being the shooter. If shooter does his part this load easily shoots 0.5 moa in both rifles. Also shot both rifles from 200 to 600 yds on steel, wasn't really shooting for groups on steel but similar results for both rifles.

Collected a little velocity data and was a bit surprised that the same load in both rifles had significantly different ES and SD.


26" Barrel

2693
2686
2697
2689
2696

AVG: 2692
ES: 11
SD: 4.7



20" Barrel

2642
2595
2629
2615
2603

AVG: 2617
ES: 47
SD: 19.1


Both sets of velocity data were collected on same chrono within an hour of each other. I realize the sample size for each is small so definitely less than perfect data but it's interesting that the trends are so different. Anyone had similar results with 20" .308? Is there a better powder choice than Varget for relatively short barrel .308? Any ideas what may be causing the velocity variation in the 20"?

 

[MitchAlsup]

Re: 308 Velocity Variation in 20" Barrel

About 80 fps is about what should be expected.

 

[Victor N TN]

Re: 308 Velocity Variation in 20" Barrel

With SDs of 4.7 and 19.1 those aren't to be taken lightly. If the accuracy on both is good, leave it alone. Sometimes people read too much into the numbers. You can have perfect repeatability on the numbers and the accuracy be off. Unless you're shooting competition with both rifles, I would leave them alone.

 

[Rob01]

Re: 308 Velocity Variation in 20" Barrel

I thought you didn't lose any velocity going from a 26" barrel to a 20"?

(For anyone not picking up on that it was sarcasm but not directed at either poster. Just at the sheeple who keep repeating that.)

Nothing wrong with Varget in a 20" .308. It's one of the best powders you can use. Length not withstanding, you have two different barrels there which have much different round counts and probably look completely different in their internal appearance so they will effect the bullet differently. Even if they had the same round count and were the same length they might be different.

Try to work with the powder charge on the 20" barrel and see if the ES drops. 45grns is the max for Varget and the 175 SMK so you can go up a little. I use 44.4grns in my 20" .308 with Lapua brass.

 

[DFC]

Re: 308 Velocity Variation in 20" Barrel

Sd's don't mean a lot at 100 yards, but at 600 it is a big deal. Short barrels in 308 normally have a higher SD than a longer barrel.

 

[Numn]

Re: 308 Velocity Variation in 20" Barrel

Thanks all for the response.

Rifles are not currently used for competition; however, I do enjoy shooting out to 1000 yds. Didn't get the chance to shoot further than 600 this week because local SWAT had reserved the 1000 yd range...great guys with more important range work than me so no problem. On the up side, 2 of us had the 600 to ourselves and the weather was great.

Looks like I'm headed in the right direction with Varget but should do some more load development on the 20". Rob, thanks for the load data. Also looks like the 20" may not equal the 26" ES and SD but practically for my use should be ok. Should be able to do some testing of various loads at distance within a couple of weeks. Thanks again for the help and I'll report back if I'm able to improve.

 

[Hummer]

Re: 308 Velocity Variation in 20" Barrel

Try this.
I have a form I made up years ago that on the top I have all the important questions, rifle number, caliber, barrel maker, type chamber, propellant number/weight, bullet type/weight, sizing of case, primer manufacturer, temp at time of firing etc etc.

On the lower half of the page I have four columns.
The first column is for first shots from cold/clean bore.(Up to 5 shots on this column which are recorded and then chrono zeroed out).
2nd Column is 10 lines for each shot and at the bottom is the min, max, SD etc. which is recorded for the first 10. The chrono is NOT ZEROED but returned to recording the final ten rounds so you have a 20 round string and you know the changes as the barrel heats up.

It is always interesting to note the ES AND SD changes between the 10th and 20th shot.

Ideally if you have a way to record your shots down range, you are really into some good data now. If I am by myself I shoot at 36" square 3/4" armor plate painted white with 3" black dot in center and I draw a plot as I shoot. Range is 600 yards.
When my buddy from England came over we did a shoot and mark on paper target and after each shot the target is marked as to shot number in the string.

After shooting you write the velocity down on this target next to each shot hole and you know exactly what velocity variation is doing for/to you.

We found it fascinating to see the relationship between velocity and vertical dispersion on the target and got lots of good data.

Even more remarkable is to have a 300 yard target set up with a aim point and blank news print with a paster on it as aiming point. The 300 yard target is placed so a shot going through the 300 yard target prints again on the 600 yard target and the relationship of the patterns. The 300 yard paper is over a big hole cut in target so the bullet only sees the thin news print as it goes through. We did maybe 50 of these tests and found the groups to more than double in size from 300 to 600 on the range of 125 percent. Yet remarkably three of the groups were less than double size at 600.For instance we found that a flier shot at 300 did not necessarly repeat at 600 and vice versa.

Now I know there are those that will say the news print paper deflects the bullets going through at 300 and this may or may not be true but this is the exact same way we did it at Aberdeen Proving Ground in sniper rifle testing except the mid range target was high on a frame at 500 yards.

If you have a better way to determine such dual range performance please do tell and explain exactly how it is to be accomplished.

 

[shoot4fun]

Re: 308 Velocity Variation in 20" Barrel

I would gather more data before making any conclusions.
Prepare both barrels the same or as close as possible. I have seen oil residue cause very big swings in chrono numbers. Watch carefully what is happening in that respect.
Also, I don't know about Utah but, one hour can make a big difference in atmospheric conditions in Alabama. No matter that Varget is said to be temp tolerant it is not fool proof. It certainly does change in the Alabama heat and humidity.

 

[MontanaMarine]

Re: 308 Velocity Variation in 20" Barrel

I lost about 80 fps going from 26" to 20.5"

Depending on how close you are to the chrono, the shorter bbl may be hitting it with more muzzle blast, and giving you some quirky reads.

I set the chrono 30 feet away most times. Didn't notice any odd increase in ES after cutting the barrel.

 

[Numn]

Re: 308 Velocity Variation in 20" Barrel

I may have been too close to the Chrono, only 5 feet and was shooting prone so there may have been some ground effect. The long barreled TRG that measured more consistent has a Near brake and the qty of side and rearward blast has got to reduce forward blast. Maybe this has something to do with the ES/SD difference between the short and long barrels? I should have a little time this week for testing and I will move the chrono further away.

I'll collect more velocity data with the revised chrono location for the 44.0 gr varget load then time permitting will also collect data in 0.4 gr increments from 43.2 to 44.8 gr. If I time this right should be able to get a couple of hours that the temp should vary less than 10 degrees.