Pressure change?

G

Goose02

Inactive
New member, been reading for a couple years. Relatively new (5 years) handloader.
I developed a load for my 35 Whelen for Moose here in ME in summer of 2018 and used it successfully! I used a Barnes TSX 200g and 8208XBR. I could not find a recipe for this bullet with this powder but I was able to find loads for other bullets. While I know that can be dangerous, particularly with the monolithics generating higher pressures, I did start low, worked up and watched for pressure signs. I ended up below the max load and had comparable velocity and great accuracy. I ended up using this load again to zero the rifle after I had the stock shortened and had a sticky bolt. First was minor second a little more and I quit.
I am trying to figure out why a load that was previously OK would be over pressure now. I have considered temp - cooler now for sure when I worked up the load. Change in OAL - remeasured and no change. I loaded fairly far off the lands too as the TSX's seem to shoot more accurately that way. I also measure case length to be sure they weren't too long and that was OK.

I am about to break them down and remeasure but I did load all of them at the same time and weighed each charge individually (25 cartridges).

Wondering if any of those here with way more experience than me have ideas why this may have happened. It certainly got my attention. I have never loaded a round above a recipe or at a velocity above what recipes state in the manuals, even taking into consideration my often shorter barrels.

Appreciate the wisdom of the Group.

Rifle is a Forbes 24B

Goose
 
Welcome to the forum.

The temperature increase will contribute to it, for sure. The solids are not just harder, which raises start pressure, but they are less dense than lead-filled bullets, which makes them longer when the base and ogive are the same shapes, so they sit deeper into the case and use up more powder space or else you have to seat them longer and closer to the lands, both of which can raise pressure. For those reasons, the original pressure could have been nearer the limit than you thought without getting to sticky bolt lift (asymptomatic excess pressure that your rifle handled well enough at cooler temperatures), so that a little increase in temperature caused the problem. Also, if these rounds of the same load were rounds you loaded back when you originally developed it, the bullets and necks tend to stick harder with age, which raises pressure a bit.

Another factor known about extruded stick powder is they can settle with enough vibration, leaving the grains packed more tightly together, which lowers the effective burn rate. In the 1995 Precision Shooting Reloading Guide, one of the authors described a load that worked fine when he loaded it at home, but which caused sticky bolt lift if he loaded the cartridges on the bench at the range. It turned out the settling of the powder by transportation vibration was the difference. So, if the original load was transported to the range with more vibration on bumpy roads, that could cause a reduced effective burn rate.

Another possibility, if the load is not filling the powder space over 90% is you were carrying the cartridges to the range nose-down in the box the first time and nose up the second time. Vibration can pack lower case-fill stick powder so it tends to stay forward or rearward in the case when fired. This will reduce or raise pressure and velocity, respectively. I once intentionally fired some 1964 National Match .30-06 (about 82% case fill of IMR 4895) both ways and got an 80 fps average MV difference and a rounded primer with powder forward and a flat primer with powder over the flash hole. Had the load been warmer, the difference could go to sticky bolt lift from none.
 
Unclenick - thanks for your thoughts.
Temperatures were actually lower when I experienced the sticky bolt. Your other thoughts are well taken, especially those regarding powder settling. Thanks also for the link - much to learn. I will check case fill but I do believe it was greater than 90%. Plan on pulling them down and measuring charge weights. Will start low and work up again.
Was starting to get a complex with so many view and no replies!
Goose
 
You mentioned that you used data from other loads for other bullets. How did you decide on seating depth? Did you use the same Over All Length in the recipe you followed?

I was intrigued by Barnes recommendation some years ago when I was experimenting with .270 rounds that Barnes should be seated at least 0.05" from the leade. There was no accompanying reasoning and I just made the presumption that solid copper bullets are more difficult to produce with the ogive in the same place, so even if you loaded them all to the same OAL to Ogive, ignoring the 0.05" recommendation, some might be deeper seated than others, increasing pressure.
 
Dr. Lloyd Brownell, back in his seminal 1965 study of pressure, showed seating deeper than contact with the lands lowers pressure, but only to a point, after which seating still deeper raises pressure again. There is a transition. Lowering pressure by increasing gas bypass around the bullet is the dominant term initially, but raising pressure by the bullet subtracting from the powder combustion space becomes the dominant term when you seat deeply enough.

attachment.php


Goose02,

I noticed that I crossed up your load data in my head with another post. I've deleted that. For your situation, the Hodgdon 200-grain data for this powder is for a short (0.873") RN Sierra bullet seated to 3.050" COL. Your TSX is 0.265" longer, from Barnes's site. If I go to Barne's data for 4064 with the TSX seated to 3.300" COL (Barnes's recommended number) and adjust QuickLOAD to get a matching velocity result, then substitute the Sierra bullet at 3.050" COL I have about 10% lower pressure. I don't know the COL or powder charge you are using, but reversing that process with 8208 XBR produces a similar pressure difference. So your charges will need to be a little lower than the Hodgdon 200 grain RN load.

There are other factors, like case capacity and so on to consider, but I would use a starting load about 5% lower than that listed for the Sierra bullet and work up from there.

What charge and COL are you using? Do you know the case water overflow capacity of one of your as-fired cases? (As-fired reflects the volume the powder sees when pressure is approaching its peak.) To find it, you weigh a fired case that still has the primer in and then fill it level with the case mouth (no meniscus) with water and weight it again and record the difference in the two weights and the length of the measured case. With that information, we can see if your case capacity is tight and also QuickLOAD can use that information to make pressure and velocity estimates.

One other thing that can raise pressure is powder deterioration. Mostly it weakens a powder, but the U.S. Navy found certain combinations of time and temperature will result in a period in the deterioration when the deterrent coatings have been preferentially consumed, raising pressure.
 
A lot of the manufacturers are generous with free data if you want it only for one or two loads. They'll ask for money for a whole manual's worth, but I've never had one begrudge sharing information on a cartridge and bullet-specific combo. Whether or not they've developed data with your specific powder choice is another matter, but what data they have they will often share.
 
cdoc - I seated at 0.050" off the lands - measured with Hornady comparator - as is recommended by Barnes as a place to start. I do measure base to ogive for each round and there was very little variation in these measurements - maybe 0.003" max - so I think the bullets where fairly consistent.

Unclenick - I was wondering why you were mentioning 30.06 but thought it was just as an example. The above information is quite interesting. I was aware of the idea that there was a transition point for pressure from decreasing to increasing with seating depth but I had never seen it expressed in a graph form. I presume this is somewhat different for each cartridge and recipe?

Load was 58.5g IMR 8208XBR at a COL of 3.28 (which was 0.050" off lands as I measured.) Nosler brass, once fired, full length sized - Case length 2.475" Fed 210 primers. I try to move the shoulder datum back 0.003" when resizing. Velocity was 2750 fps out of a 24" barrel.

Case capacity seems to be 74.2g H2O which seems high as Nosler manual states 63.3g.
Length after firing 2.483". Measured the two I fired that had the hard bolt lift. The primers do not seem to be flattened, but my impressions is that finding is not very useful. I do not see ejector marks or swipes on the case head.

I did start low but not 5% less -started at 56g.

As far as powder deterioration - it has been stored closed tightly in a climate controlled environment and it has been about 2 years since it was purchased. The rounds themselves have also been in the same environment except when taken on the hunt - cold - and to the range.

Old Roper - right. That would have been an easy way to be sure of the load for that bullet. I will check with Barnes.

Appreciate everyone's help on this.

Goose
 
Did I not reply correctly?
Wondering if my last post went through.
Still wondering why these cartridges that were fired and didn't show pressure later resulted in difficult bolt lift.
I am also now puzzled by the case capacity measurement I got above as it seems to be too high.
See people are reading but not responding - hope it's nothing I said!
 
You must log in or register to reply here.
Back
Top