Thoughts on Barrel length for the 7mm-08 imp.

#4
Re: Thoughts on Barrel length for the 7mm-08 imp.

I think what I am going to do is some testing, I will sacrifice a barrel, and just go inch by inch and see what kind of velocity loss I experience as I go shorter. Pending the results of the 20", I may even go down to 18". Find a length that is acceptable, and run with that in my other barrels.
 
#5
Re: Thoughts on Barrel length for the 7mm-08 imp.

Cutting down 1" at a time, then doing load development at each length practically guarantees that by the time you're done testing, the barrel will likely be washed out deader 'n dead.

If you're gonna cut it, do it once, and do it short, develop a good load, then do the same for a full length barrel. Then think long and hard about the third one.

At least this way, you end up with three still-usable barrels. And admit it, if you had an 18" barreled 7mm-08 AI, you wouldn't kick it outta bed, would ya?

Greg
 
#7
Re: Thoughts on Barrel length for the 7mm-08 imp.

The same load all the way down won't tell you diddly about potential performance at each length; only a load that matches each unique harmonics solution will tell you anything meaningful.

That's why I advise skipping the intermediate steps.

The two extremes will tell you relative velocities, etc., then you can cut the third to approximate a targeted velocity, and accuracy them becomes a matter of load development. If you really like either better, you have the option of cutting the second long one to either length, and 90% of your load develpement is already done. Mainly, you get away with two rounds of load development, and just a smidge more.

Greg
 
#8
Re: Thoughts on Barrel length for the 7mm-08 imp.

I would like to see how the 18-20" performs as well. I'm thinking my next build will be either a short 7-08 or possibly 6.5 Creedmoor.
 
#9
Re: Thoughts on Barrel length for the 7mm-08 imp.

I'm tossing the idea of regular old 7mm-08 in a short barrel for 2 reasons. if I could get the 162 amax to 2500 fps it's still supersonic well past 1K and I might be able to size my existing .308 brass down.

anybody using resized .308 brass for 7mm-08? did you have any issues with the resizing?
 
#10
Re: Thoughts on Barrel length for the 7mm-08 imp.

Have used .308 brass for .260, sizing in two steps, 7-08 and then .260. For the .260, neck turning would almost certainly be necessary, maybe not so for the 7-08.

Actually, I've always found 7-08 brass to be more available than .308, especially in time like ours right now, and use either it or .243 to make my .260 when I can.

Rem .260 brass is also fine, and might make a good source to neck up to 7-08, but for me, Rem brass is not as readily in-stock available as Win, and Win doesn't (I think) make .260 for sale as virgin unprimed brass.

Greg
 
#11
Re: Thoughts on Barrel length for the 7mm-08 imp.

I am planning on using Fed brass and hopefully some Lapua brass myself, will size down from .308 on both accounts. If I have to ream the necks a little to get it done, that is just not a big deal. If I get serious with it all would likely want to control neck tension anyway. If not wanting to weigh cases and turn necks, then any commercially 7-08 brass should be just fine.
 
#12
Re: Thoughts on Barrel length for the 7mm-08 imp.

Greg,

I have been a lot more thinking on the shortening and working up loads at each lenght. My question is this: Do you think if I took the time to do a ladder test, and find the sweet spot at each length, that the results will apply to all 7-08 improved barrels at the same length? or would it only be applicable to that particular barrel? The reason I am asking is because if I am going to put the effort in, I want to do as complete a job as I can. If I cut back in 2" lengths and do a ladder test at each length (using a single chosen powder and bullet), then I would be somewhere between 150 and 250 rounds down the tube. That hardly seems to be really detrimental to the barrel life, and would be a tremendous resource for others looking at the 7-08 imp. All that said, if the results won't translate to other barrels, then it is basically probably not worth the effort.

Thanks

Dave
 
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#13
Re: Thoughts on Barrel length for the 7mm-08 imp.

I think it would be a hit or miss since no two barrels are identical. However I do think it would be a great resource for others going to 7-08 imp. I don't think the loads you work up would be that easily transferrable to another without some slight tuning involved, but once tuned I bet it would be very close. It doesn't seem that 250+/- down the tube would be detrimental at all as long as it was done properly. I do hope you decide to do this though. Your results would definately help me decide between this and the Creedmoor.
 
#14
Re: Thoughts on Barrel length for the 7mm-08 imp.

I have limited experience with this, but I can say what I say with confidence.

My matched pair of L-W barrels, made in the same run, are very close, but not truly identical, load development wise. They were load developed separately, and actually do show preferences for two different loads, .1gr apart (43.8gr and 43.9gr of H4350 with 142SMK's, .260 rem).

My point is that no matter how hard you worked to make such close barrel length increments work, the progression of throat and bore wear, and maybe crown variances, would likely make the results only partway reliable.

IMHO, better to go immediately to the shortest reasonable length, get your data, compare it to the full length, draw a straight line graph, and project a barrel length for the desired performance. Cut barrel two to that length, test it, and make a decision about the remaining full length barrel. The point is, the process involved with doing all the cutting and testing is not really guaranteed to render reliable results about any of th intermediate lengths, because more is changing with the process than just the barrel length.

Greg
 
#15
Re: Thoughts on Barrel length for the 7mm-08 imp.

I agree with Greg for the most part.

Do you really think the throat and bore wear wold make that much of a difference with a small round count like that? I'm not second guessing you here, I'm really just wondering.

If so I guess the only way to do a dead accurate test like that is to have a dedicated barrel for each length. In my eyes that would way more effort than it's worth, and costly at that. If you wanted to do a shorter ladder test you could do one at 20" then 23" and finally at 26". It wouldn't get the exact results you are looking for, but it would get you somewhere.
 
#16
Re: Thoughts on Barrel length for the 7mm-08 imp.

There are definately going to be variables, but I think the data set will still be valuable to people considering a build of this caliber. The crown, exact sweet spot, and throat wear will all be subject to some variability per each individual barrel, but overall, the velocity range, and likely the sweet node range will be close for anyone considering. This won't be the end all of tests, but I feel it will give me a good table to work from when I am done. I think I will use two different powders, and use 1 type of bullet, and see what the results are. 3-shot groups. Any ideas on what powders and what bullet to use? Input still appreciated. Still plan to go ahead and cut down at 2" lengths until I get to 18" that will give a great reference table. This is also going to be great practice for my machining.

Dave
 
#18
Re: Thoughts on Barrel length for the 7mm-08 imp.

Maybe maybe not, but that's not the point; it might, and that's the point.

If there's an error in this thought process, it's that the results can probably never be considered definitive, only valid within a given statistical set. Another would be that the results would be repeatable. No such guarantees apply. Each instance is a separate case, because we can only truly control a portion of the variables (for instance, can you guarantee the metallurgy is truly a constant for all the barrels? Even the maker can't do that). Mathematical calculations use constants that make assumptions; assumptions that things like uniformity and reliability exist. They don't.

So, don't look for absolutely ideal results, look for good ones, and invest your major effort into duplicating them with a decently close approximation.

The cost of perfection is not only prohibitive, it's unattainable; because perfection simply never existed, and won't be doing so anytime soon.

Be satisfied with 'good'.

Greg
 
#19
Re: Thoughts on Barrel length for the 7mm-08 imp.

I agree Greg, but doing this test in 2" increments then charting the results and compare the original to the shortest in a straight line would also be a way of somewhat verifying the validity of the steps. I guess we will see, the main result sought is to get a rough idea of velocity levels at each length, it may never be the definitive answer, but it will give me much needed practice and I will enjoy the project.

I plan to use either the 168 VLD or the 162 Amax, as those are the two bullets that I will want to use in the end rifle, since those are two of the choices that will make the 7-08 improved stand out of the crowd. Unless someone wants to make a Scenar or VLD in the 140-145 range
wink.gif


DD
 
#20
Re: Thoughts on Barrel length for the 7mm-08 imp.

At 6mmBR the load map is dominated by 4064, followed by Reloader 15. They show the 162 being pushed by 40.3 grains of 4064 for 2670fps. This isn't the improved of course, but it might be a good starting point. You may have already looked at this but here's a link.

7mm Cartridge Guide

It's not a ton of info, but there's a little there.
 
#22
Re: Thoughts on Barrel length for the 7mm-08 imp.

Your're very welcome, just keep us updated whenever you get started. I'm very interested to see what the results look like.
 
#24
Re: Thoughts on Barrel length for the 7mm-08 imp.

I'm interested in the results myself. I'd also be curious as to the accuracy potential as the barrel is shortened.

When I cut my .308 down to 20" from 24" I noticed it seemed to group about .15 MOA better at 100 with 168 GMM. I don't know if the came from the shorter stiffer barrel or a better crown. I cut the crown in a 3 jaw then used a .30 cal crown chamfering tool to finish it off. so it's definitely not the best crown in the world nor the worst.
 
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