This is a discussion on KKM, Storm Lake, or stock within the Polls forums, part of the Smith & Wesson MP Forum category; Thursday was my range day. When I got home from the airport I loaded up some mags with some higher performance reloads. Supersonic 180 gr. ...
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|September 6th, 2013, 04:00 AM||#16|
Join Date: Oct 2007
Thursday was my range day. When I got home from the airport I loaded up some mags with some higher performance reloads. Supersonic 180 gr. FMJ loaded with Longshot.
Shot five rounds with the 1 in 10" barrel (we're selling it as part of a package deal over in the "For Sale" forum) and 5 with the 1 in 18.75" twist barrel. All 5 shots were supersonic with the 1 in 18", but 4 were not with the 1 in 10". Couldn't find my crono, as we're in the process of moving, but the velocity was obviously lower. I did it 2 more times with similar results. My 1 in 18" was invariably supersonic, which is not unusual at this altitude, while the 1 in 10" was usually subsonic. My guess is they were averaging 2.5 to 5%, or about 30-60 fps. slower through the 1 in 10" barrel.
Checked the brass once I got back to the house and the rounds shot through the 1 in 10" also had some primer flattening, at least more than the others.
While this is not conclusive, it seems obvious to me that the 1 in 10" barrel twist is just a little lower velocity, with slightly higher pressure, than the 1 in 18". This is not necessarily a bad thing, but reloaders switching from a 1 in 18" barrel twist to a 1 in 10" might want to drop a grain off their maximum loads until they're sure.
Last edited by KRWeiss; September 6th, 2013 at 04:21 AM.
|September 6th, 2013, 04:47 AM||#17|
Join Date: Oct 2008
Location: St Petersburg, FL
I'd LOVE to see someone test the two barrels from a Ransom Rest completely independently... With 115, 124, and 147Gr bullets...
I'm absolutely convinced you'd see ZERO difference in accuracy, even out to 25-50 yards. The influence on spin rate of PISTOL bullets, down around 1000 ft/sec and less, inside 25 yards, has got to be so negligible as to be ridiculous.
You'll find more variation from one barrel to the next, and one brand of ammo to the next.
|September 6th, 2013, 03:43 PM||#18|
Join Date: Jun 2012
Location: Terrace, BC
Jeff maybe but explain why we see tumbling within 10 yards using Lyman's 356637 147gr lead bullets that run prefectly fine in CZ and STI guns with the faster 1 - 10 twist.
I also wonder why the collective minds of S&W decided to go with the 1 - 10 twist over the 1 - 16 twist.
Here are some chron results using different guns with different twist rates but the same bullet and powder combo.
Lyman 356402 124 gr 4.1 gr of 231 OAL 1.115
Avg vel 1091 Hi Power 1- 10 4.6" barrel
Avg Vel 1129 M&P 1 - 18 4.25" barrel
Avg Vel 1088 Tanfoglio 1 - 10 5" barrel
Avg Vel 1090 Trojan 1 - 10 5" barrel
Avg Vel 1083 CZ 85 Combat 1 - 10 4.7" barrel
In all the guns the loads are extremely accurate. Increase friction may account for the slightly faster velocity achieved in the M&P but there is no practical difference in overall performance evidenced in shooting off a rest or free hand while playing IDPA or IPSC. Whatever the pressure increase was it did not provide any physical evidence on the primer face.
Loads have been tested over a number of years under various temperature conditions. All consisted of 10 round averages shot over a F1 Chrony.
ps I have not developed loads for the 147 gr bullet with the various guns as I have been shooting the M&P platform for the past five years to the exclusion of the other guns listed and the results with the heavier bullet in the M&P lead me to stick to the 125 gr bullet. It works for IDPA and IPSC.
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|September 6th, 2013, 05:31 PM||#19|
Join Date: Oct 2007
I was being conservative guessing it was a 2.5 to 3% drop in velocity when I went down to the 1 in 10 barrel twist. Accounting for barrel length, your crono results show a full 4% decrease in velocity. May not mean much to you, but to a 9mm major shooter it would mean compressing even more powder into the case and dealing with even more pressure just to make PF, but they're already out at the bleeding edge of the pressure envelope as it is.
|September 6th, 2013, 11:26 PM||#20|
Join Date: Aug 2013
Location: Altamonte Springs, FL
That's crazy, I never expected the velocity difference to be that dramatic. Tell you guys what, buy me a ransom rest and I'll shoot every combination of twist rate, barrel length, and bullet weights I can find and report back grouping information :wink:
Definitely not sent from the can using Tapatalk
|September 9th, 2013, 06:36 PM||#21|
Join Date: Jun 2012
Location: Terrace, BC
I agree the difference is greater than I would have expected but I am not sure it has much significance in the practical application the 9mm guns are designed for. Paper targets certainly wouldn't notice the difference nor, I don't think, would a human lung .
|July 11th, 2014, 01:44 AM||#23|
Join Date: Jan 2012
|July 11th, 2014, 04:16 AM||#24|
Join Date: Jan 2008
Location: Republic of Texas
You need to get your eyes checked for glasses
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