.17 HMR barrel cut

[dwalk]

gendoc did a barrel cut on the 17HM2 and was satisfied (?)

could the same thing be done to the .17 HMR with satisfactory results? no loss of accuracy/velocity?

[notnodak]

I believe that a 17"-18" is optimal length for velocity on the HMR.

[quickdtoo]

Here's Bullberry's test data.

Tim

http://www.bullberry.com/HMRdata.html

[dwalk]

thanks Tim...tells me what i want to know...

[gendoc]

 during my crowning process, test firing for accuracy adjustments also
showed faster results of speed. same ammo each time.

notice that bullberry did not say they crowned after each chop.

in my notes i show about average 22 fps faster when inside 1" group @ 50yrds
than 2"

then average 17 fps faster with 3/8" group @ 50yrds

not much faster when making 5-shot one hole  @50yrds.

i was shooting from a strapped lead-sled in a test cell.

from the beginning to the end, i would polish the crown after each shot group
until i got what i wanted.

[J. Plate]

Bullberry has quite an interesting logo....   



      

Looking at that data, 19" had the highest average velocity, but 15" was the most consistent with a deviation of only 27 fps between the shots. 

I would have to ask at what value that "spread" leads to accuracy robbing inconsistency.

[djw]

Looking at that data, 19" had the highest average velocity, but 15" was the most consistent with a deviation of only 27 fps between the shots. 

As stated above, 17-18" looks optimal. You only lose 16fps compared to the 19" barrel, with a spread half the size.  A 15" barrel on a Handi will most consistently give you a trip to Club Fed.

[J. Plate]

It's all hypothetical anyway.   My barrel is a 22" bull.   Independent of what that "study" says, when I'm putting 10 rounds in a group under the size of a dime at 100 yards, I'm not even thinking about cutting my barrel!   LOL

Not to mention the fact that I'm in a research field, and 5 shots is hardly a sample size.     A couple hundred shots per barrel length would give much more statistically significant results.

[gendoc]

before i chop'd my m2, i prolly had over 600 shots thru her. and i was pleased
with the results. after the chop, i got over 500 thru her and the statistical significant results
were much better....
but no 2 shoot'n irons are the same, kinda like the operators !!!!

[gcrank1]

J.P., Since it is your field perhaps you can verify something I read years ago regarding test groups (and yes, the more shots the better, but practicallity raises its head).
The author stated that 5 shot groups are not very meaningful and 10 shot groups are much more so, but that 7 shot groups will tell us most of what the 10 shot would, at less time and expense.
I believe the context was in determining which reloads were worth pursuing further and would hold true for choosing factory ammo for one's barrel.

[J. Plate]

It really depends on what you're trying to find and the data set.   In any situation, the more samples you can take, the better.  You may find that 7 samples in a given situation gave you the same statistical significance as 10 samples for a single test or data set.   But again, you're only talking about increasing your sample size by 3.  It's not so much 7 vs. 10 being the issue, but only having taken 3 more samples.   Unless one of those 3 were a serious outlier, you would always find that 3 more samples isn't going to give you a stronger result.   Saying 7 is better is relative.  Those extra 3 samples would be negligible if they were Samples 8, 9, and 10, or if they were samples 101, 102, and 103.  And obviously 100 samples would be better than 10, but I wouldn't say 7 is better than 10, only because they are so low a sample size anyway.

5 samples/shots seems like a good test to narrow in, but I'd like to see a follow up test from someone with the capability to cut and re-crown barrels.   Take the top 5 or 6 barrel lengths and run 50 to 100 shots per barrel.   With a 50 round box of shells in my area running about $14, the total cost in shells would be less than what you would make selling a single rifle.   Meaning, if you got just one more person to buy a rifle based on the data, it already paid for itself.

The problem that I've always seen with tests like this were mentioned two posts beforehand.   Different barrels, different shooters.....all factors will produce different results.  When you add things like rate of twist, bullet stabilization, and other environmental factors to the mix.....this type of study is highly subjective.

[gcrank1]

Thanx for your prof. insights.
I think that the crowning could be eliminated for such a test. The cut must be clean & square and no burrs in the bore, but otherwise the crown only protectes that bore from damage in use. I base this on my false muzzle rifle which type has no crown and has been regarded as the ne plus ultra for many years.