Strange 308 Load performance

[MVW]

Gents:

I have recently started a couple of threads looking for help on my reloading technique. I have been pretty careful and consistent with my reloading, but usually end up with large SD and ES. This past weekend I did something a little different that seems to suggest that it is not the reloads, but rather the shooter or the gun.

I should say that I posted this on Benchrest Central, but I am hoping to get a broader response.

The day was overcast, calm and 47-51 deg. I shoot a Savage F/TR 308, but I was shooting off a bench with a Hart rest. I warmed the barrel up with 5 rounds running about 2,720 fps. I then went and set up my CED chronograph.

I then shot 20 more rounds at 4 different powder charges as shown below. Note however, the sequence that I shot these in.

43.2 43.4 43.6 43.8 (VARGET, 175 GN SIE MK)
2,723 2,716 2,735 2,738
2,734 2,730 2,744 2,749
- - - -
2,740 2,737 2,757 2,751
2,748 2,744 2,756 2,762

I shot the first flight [43.2 - 43.8]. I took my time shooting, but didn't specifically wait a long time between shots. I then shot the second flight. Two patterns are clearly developing (described below). Then the clouds came in and the chron wouldn't register, so I took a break until a cease fire was called (~15 min) and then went out and took the screens off the CED. I then fired the 4th and 5th flights.

Two things are evident from the data: first, 43.4 is consistently the lowest of the four powder charges, but second and MORE IMPORTANTLY from an ES/SD perspective, the speeds increase over time at each load!

I should say that the barrel was warm to the touch, but not hot like you would get when you put 20 rounds down range in an F-Class shoot on a hot day.

Does this make sense to anyone?

Thanks, Mike

 

[MrMajestic]

The expansion coefficient of steel is 6.5 PPM per degree Fahrenheit. So for every degree your barrel increases in temperature your bore is enlarged .0000065" per inch. This should reduce friction therefore an increase in velocity would seem normal. Unless you are shooting extended ranges the difference would be difficult to measure. How are your groups?

 

[Laurie]

Mike,

three points.

1. While your ES values could be smaller, they're not terrible by any means running in the 22 to 28 fps ranges over the four loads despite the pause in mid shoot effect. You don't say what your brass or primer is and how you're setting neck tension.

2. You're assuming the chronograph gave 100% consistent readings despite a large change in the light and your having removed the screens afterwards. A 10-20 fps change in chrono' readings is very small indeed in relation to 2,700 fps + values and you've no way of knowing how the accuracy is affected by external changes, or for that matter what tolerances the chrono works within even in ideal steady light levels. More than a few people are now moving onto the infra-red CED kit for outdoor use to reduce ambient light level induced inconsistencies.

3. The interaction of barrel throat and bullet make / materials / design can see two or more makes / models of bullet of a given weight give markedly different ES values with the same powder and optimised worked-up loads. Switching to the Berger 175gn Long-Range BT or Hybrid models (say) could very well produce a significant change in the ES using the same components and at very close MVs.

You don't say how the combination is grouping or at what distance you'll use it. If it groups well I wouldn't worry about those ES values, especially if the ammunition won't be used beyond 600 yards, 800 tops. If you plan to use this combination at 900 and 1,000yd, I'd worry FAR more about a combination of a low (0.243 G7 average) BC bullet and MVs below 2,750 fps. Under standard ballistic ambient conditions (59 F / 29.92" Hg pressure), the terminal velocity is calculated as 1,162 fps at 1,000yd, barely supersonic and very much in the transonic turbulence zone that can see funny things happen to bullets in relatively small wind changes. (I know - been there and got that T shirt with the 175gn SMK at 1,000!)

 

[MVW]

Mr. Majestic:

Thanks for the comments. This was a bit of an academic exercise, but I thought that I had noticed generally increasing speeds throughout a shooting session. This experiment really struck me that I wasn't imagining the effect.

I have never seen anything written on this topic and wonder if this is something that has been discussed? I did notice at an F-Class shoot last summer that I kept dialing down the elevation as the day heated up, but didn't put it all together.

Groups are not so good, but at Boyd Allen's suggestion, I am sending the gun off to get rebedded.

Mike

 

[amlevin]

A couple of thoughts---

It may well be that changing light could give you some variations. I purchased a chronograph that uses infra red sensors because the lighting at my "home range" changed from minute to minute and was driving me nuts.

As Mr Majestic stated, as the barrel warms, the friction characteristics change thus a change in speed. Don't dismiss the temperature effect on the round itself. If you take a round from your tray, chamber and shoot it without delay, the powder is at one temperature. If you chamber a round and then sit and wait for any length of time for a certain wind condition, the powder is now much warmer. For consistency I find I get better "readings" when I hold the rounds in the chamber for the same amount of time and try to space shots by the same amount of time. If your subsequent flights were shot at a different "rhythm" than the first, then there will be some variations due to cartridge temp at the time it's fired. How big? Who knows but it's another variable that doesn't hurt if it's removed.

 

[markm87]

Mr. Majestic:
Groups are not so good, but at Boyd Allen's suggestion, I am sending the gun off to get rebedded.

Do you have a guy who can look at the gun first? Bedding is always good, but you might find a barrel issue.

Our Riflesmith is pretty good about spotting a barrel that will never shoot.

 

[MVW]

Laurie, Amlevin and markm87,

I agree with your comment about the chony. I fussed about this as well, but I bring this up because it seems to be a recurring theme, not something specific to that day.

I do have both Berger 175 and 185 and am planning to load those for longer range, but I was shooting up the SMK and this popped out. It seemed peculiar enough to make it worth asking about.

I agree it could be the barrel, but enough folks have suggested that I have the gun rebedded that I am going to do that before I do anything else. It may not answer the rising velocity question, but perhaps it will improve the groups.

I don't have a riflesmith that I would trust to answer that question.

Mike

 

[Laurie]

Mike,

another thing to check is bullet dimensional consistency. Recent Sierras are generally good, but go back only a few years and MatchKings, or at any rate some batches, had a reputation for large variations in their ogive-to-base measurements.

Although the 175gn SMK is a marvellously tolerant design vis a vis coping with the amount of jump it makes into the lands, if there is a lot of variation between bullets in these dimensions, it can also affect MVs slightly, enough to increase your spread a little, or even more than a little.

 

[MVW]

Laurie,

Thanks, and yes, I measure and have by sorted by every conceivable dimension of the bullet and seat to a measured distance of 0.010 off the lands - but I have tried other depths as well.

Do other shooters find that their speeds increase like this? Perhaps this is common and it just isn't mentioned by people, but shooters publish examples of very good stats and I have seen some examples of good targets and data, so my situation may not be that common.

Mike

 

[jo191145]

I've seen that happen just once,,,, but on a larger scale than you've noticed.
Also happened to be a 308W Kreiger Palma barrel.

Trying the 8208XBR with 155's. Shot an Audette ladder. Found a tight node up high. Some three shot groups to verify and things were looking good but with slightly raised velocitys.
I find it rather common that Audette ladders and subsequent groups do not always match velocity consistently. No big deal thats why we tweak.
Picked the tightest group from the test and loaded five. Now I recorded roughly an extra 40fps over an already higher velocity. Every shot increasing steadily.
Saw a pattern that I thought I could improve upon. Loaded five more .2gns less and .005" further from the lands. Still an incremental increase in velocity.
I dropped another .2gns and held seating depth
Every shot still increased until by the end I was a full 200fps faster than the original load. TEST OVER.

What caused this? I still don't know. Many may speculate that I was overcharged and the reduction allowed an increase.
Personally I blamed it on crappy 8208 laced with too many chemicals.
I know some have found pleasure in it but I consider it one of the biggest wastes yet to be bottled and sold.
That was the fourth rifle I tried plastic based powder in and none liked plastic.
Sorry I can't help but thought you may like to know.

Try any other powder besides XBR or any cousin of XBR (Hodgen Extreme line) see if it happens again.