Revolver Velocity Variations >30fps Average

[robomanusa]

I was chronoing some new loads a little bit ago using titegroup and 230gr LRN(actually vary between 232 and 234gr) in my 625 revolver, I first started firing the strings single action across the chrono then sent one thru it double action and got a severe jump in velocity im talking over 50fps diference. I thought maybe a fluke???

So I tried a few more single action went thru at 710 average......then a few double action and they went across chrono at 750 average

wtf! ? any ideas?

[George]

Is it possible that you are tipping the gun back when you cock the hammer for single action and keeping it level when you shoot double. This will pour the powder back to the rear of the case right over the flash hole. This is known to cause just about that range of FPS increase because the initial burn front is a little faster this way and the pressure spike is a tad earlier and higher because of this. Each shot fired will force the powder forward in the cases again so a follow-up DA shot will not exhibit the increase. As soon as the gun is tilted rearward a bit again (hammer cocking?), the powder goes that way again.

Try keeping the gun absolutely level for every shot, or even tipping it back drastically and deliberately for each shot to see what is up.

--

Regards,

[robomanusa]

I get the higher velocity when shooting double action, meaning the powder isnt at the rear of the case, when I shoot single action im sure I tilt the gun back a little bit......I came home to load up some more test rounds as I ran out, LOL, so after I get a few more loaded up I will run back out and test again, I may have to bump the load a tenth or 2 to make sure if makes major if gun is fired single action mode or just make damn sure the chrono guys fire the gun double action at the match...LOL

[George]

Do run the test where you tilt the gun forward and fire a shot, repeat several times. Then repeat tilting back using the same hammer drop style and I will bet you do see a velocity change of between 30 and 60 fps. Then try the other action method both ways. This should give you enough data points to pin the blame somewhere ;-)

Let us know the result.

I am also wondering if your wheelgun offers a difference in spring force to the hammer depending on how it is tripped. This difference, if present and great enough would definitely ignite the primer a little sharper and could be responsible for a slight increase in flame front propagation inside the cartridge. Rounds that have a small powder charge volume in relation to case size are well known to produce non-linear chrono results depending on powder position variance and the resulting flame front variances that result.

--

Regards,

[robomanusa]

OK, here are the new results...

Gun Tilted up before each shot fired single action:

#1- 728

#2- 734

#3- 747.1

#4- 724.2

#5- 741.7

HI: 747.1

LOW: 724.2

ES: 22.9

AVRG: 735

Gun tilted down before each shot fired single action:

#1- 733.5

#2- 706.7

#3- 740.4

#4- 730.1

#5- 719.1

HI: 740.4

LOW: 706.7

ES: 33.7

AVRG: 725.9

Gun held level as can be fired double action:

#1- 757.2

#2- 766.4

#3- 768.6

#4- 764.9

#5- 752.9

HI: 768.6

LOW: 752.9

ES: 15.7

AVRG: 762.0

Gun held level as can be while cocked and fired single action:

#1- 735.2

#2- 701

#3- 737.5

#4- 704.7

#5- 709.7

HI: 737.5

LOW: 701

ES: 36.5

AVRG: 717.6

All shots were using my S&W 5" 625 with chrono set about 15-16 feet away or whatever the length od the wires were on the pact Mark4

load was 4.1gr of titegroup

federal primer

230gr LRN Vance bullet

1.240" OAL

Winchester brass

[George]

The only strings without low speed flyers are the double action strings and that says that the hammer ain't falling with the same oomph in SA. The SA tilt string says that powder position improved the SA V but still allowed a flyer so powder stacking is in play but ignition is weak. The SA hammer fall force is the issue here IMO. Weak primer strike in SA mode exacerbates the random powder position in the case without tilting before each shot.

(in actuality you would need a lot more than 5 rounds for each sample to actually say anything with authority if you were serious about your data).

--

Regards,

[robomanusa]

The double action hammer fall isnt near what what the SA fall is, I understand I needed more per strings, I went back out this evening just to recheck quickly what I saw the first time and to get a few numbers for you to see quickly as I didnt write any numbers down from my first outing