Something strange happened today

[racesnake]

I went to the range today to test H4831 behind 115 DTacs in my 240 NMC. I loaded 10 rds. each of 42.0, 42.5 and 43.0gr. of H4831. Bullets were touching the lands. I shot 2 5shot groups of each loading and averaged the results. The results were as follows:
42.0gr.-----2941fps w/ ES= 10fps------0.791"group
42.5gr-----3002fps w/ ES= 8fps--------0.401"group
43.0gr----2998fps w/ ES= 11fps-------0.905"group-------slight primer cratering
My question - how come there was not and increase in velocity with an increase of 1/2gr in powder? I've never seen this before!

 

[rogn]

Not an expert, but Ive heard that when you pass the "sweetspot", you may loose velocity, pressure escalates and accuracy leaves. Sounds like this might be it.

 

[243winxb]

Increase in powder charge gets unexpected velocity. Look for an orderly progression of muzzle velocity vs. charge weight. If muzzle velocity stops going up or actually goes down, or if it goes up too much, you have a problem. The first two indicate steel is stretching. The stretch may just be due to uneven bolt lug contact, or it may mean you are stretching the receiver and fatiguing the steel abnormally. Unexpected velocity increase indicates unexpected pressure increase. With any abnormal velocity, you should back the charge off 5% from where it started. If, based on manuals, the load and its velocity seem too low for this to be happening, get your gun inspected or bolt lugs lapped and try working up the load again.

Came for here > http://www.shootersforum.com/handloading-procedures-practices/58763-pressure-signs.html

 

[SDDasher]

My testing indicates the former not the latter is the case. You have reached or passed a "sweet spot" in the incremental increase of powder. Do some .01 grain testing around this decrease and see if you don't find some excellent groups. That has been my experience. Usually within the pressure curve you could find as many as two or three such decreases or swee spots.

 

[racesnake]

The 43.0gr load did not show excessive pressure, to me. Bolt opened normally, no extractor marks on brass, primers were not flattened, plus primers are not magnum . The firing pin has not been bushed and normally shows some primer cratering at operating pressures.

 

[jcampbellsmith]

42.5gr-----3002fps w/ ES= 8fps--------0.401"group
43.0gr----2998fps w/ ES= 11fps-------0.905"group..

This difference in the two velocities above is not statistically significant. I bet if you did the test again with a larger number of shots per loading you would get a different result. The second unrelated comment I would make is that there is nothing magic about powder weights being integers or half integers. Your ideal powder load could be 42.7gr or it might be 42.4gr or...
Regards JCS

 

[gme]

How long is your barrel? You may have just exceeded the effective length of your barrel relative to the powders burn rate.
Gary Eliseo

 

[racesnake]

You could be right, Gary. H4831 is a fairly slow burning powder. Barrel is a 30" med. Palma.
Back a week or so I posted a question asking for load info for a 115dtac/h4831 combo. I didn't get any response so this test was to explore the limits of this combination. I'm extremely curious about this loading due to the low ES and accuracy potential.
Dave

 

[FroggyOne2]

When I was shooting the 240NMC, I was using N150 as main powder. But with the 115's I would be tempted to try 2000MR, N550, H4350 or RL17 as well.

 

[racesnake]

Froggy...What was the velocity with your 115/n150 load? I've been looking for some RL-17 to try but can't find any locally.
Dave

 

[wapiti25]

Shooting a small sample can give you funny results. We as shooters can not really shoot enough rounds to get absolute accurate numbers. Shoot more at your best group or repeat the whole test on another day would give more confidence in what you get. Shooting groups with smaller powder increments (.2 or .3gr) will give more confidence in your test. I like to repeat tests on different days to make sure it is good data. It is like the "blind squirrel finding a nut" or the wallet group that is a one time thing.

 

[racesnake]

I don't think you can explain away this anomaly with statistics. The above post by Gary Eliseo probably explains it best.

 

[DougMH]

I went to the range today to test H4831 behind 115 DTacs in my 240 NMC. I loaded 10 rds. each of 42.0, 42.5 and 43.0gr. of H4831. Bullets were touching the lands. I shot 2 5shot groups of each loading and averaged the results. The results were as follows:
42.0gr.-----2941fps w/ ES= 10fps------0.791"group
42.5gr-----3002fps w/ ES= 8fps--------0.401"group
43.0gr----2998fps w/ ES= 11fps-------0.905"group-------slight primer cratering
My question - how come there was not and increase in velocity with an increase of 1/2gr in powder? I've never seen this before!

Chronographer brand?
Your 43.0 gr may constitute a compressed load?... which may be causing something?
The round could beleaving the muzzle before all the powder is burned?
Barrel and chamber heat?
Accuracy of your power scale?
5 rounds seems like it ought to be enough to check.

I guess I'd say there may be many variables that are unaccounted for and I would look at the groups rather than worry about those stats.

I've seen something similar when people have posted spreadsheets on "neck tension" when using the K & M arbor with force measurement. Rounds with 30lbs of neck tension have a higher MV than those with 20lbs. Then vice-versa, the 30lb neck tension has a lower MV than the 20 lbs.

There are so many variables in this testing that it would be easy to shoot out a barrel to cover all bases. I've been there.

Here's a funny story. The rifle described in my signature. I think I tried everything and was getting 1 MOA at best. I made a mistake and bought some .308 "Lapua" cases that use the small rifle primer. I didn't use them for the longest time but then decided to try them. The first thing that happened was that my standard .308 de-capping pin stuck in the small flash hole. So, I drilled out the flash hole to large rifle size. I used CCI #400 small rifle primers. I used 45 gr Varget with 155.5 Berger BTHP (non-VLD) rounds with a COL that put the ogive on the lands. I expected nothing and got a 1/4 MOA rifle out of it (with the 155.5s). I kid you not (see targets below with 155.5 and 185 Bergers). I can only guess that the small rifle primer is giving a more consistent powder burn.