Jack Suber Posted May 10, 2008 Share Posted May 10, 2008 For those of you loading Super Comp, does Super Comp generally produce lower velocities than Super brass with the same loads? I just tried Super Comp in an open gun of ours with the same load as our Super load. I had a significant differrence in velocity (lower). Our super load that runs 169 did not make power factor with Super Comp brass (new Starline). Just curious if this is common. Just curious. This Open thing is new to me.... Link to comment Share on other sites More sharing options...
jostein jensen Posted May 10, 2008 Share Posted May 10, 2008 if it was a different gun with same load, don't be surprised of different velocities. No barrel is the same. Even two guns coming off the line after one another can have 100 fps difference with the same load from the same powder/brass/bullet batch. Link to comment Share on other sites More sharing options...
XRe Posted May 10, 2008 Share Posted May 10, 2008 (edited) I think Jack is talking about shooting the same load in the same gun, just changing the brass, not trying the same load between different guns? Usually, you'd expect it to go the other way around - SuperComp has a smaller case volume than Super, so the same charge should result in higher initial pressure. However.... sometimes high pressures (read: real high) can cause a loss of velocity - that's obviously a tricky and possibly dangerous situation. What load are you loading, Jack? ETA... make sure you triple check that all the usual parameters are the same - same crimp, same seat depth, etc... Edited May 10, 2008 by XRe Link to comment Share on other sites More sharing options...
Jack Suber Posted May 10, 2008 Author Share Posted May 10, 2008 (edited) I think Jack is talking about shooting the same load in the same gun, just changing the brass, not trying the same load between different guns?Usually, you'd expect it to go the other way around - SuperComp has a smaller case volume than Super, so the same charge should result in higher initial pressure. However.... sometimes high pressures (read: real high) can cause a loss of velocity - that's obviously a tricky and possibly dangerous situation. What load are you loading, Jack? ETA... make sure you triple check that all the usual parameters are the same - same crimp, same seat depth, etc... Dave, Yep. I am shooting out of the same gun. 8.8 grns of 4756 with a MG 124 CMJ loaded to 1.242" . All I did was switch brass. Same gun. I thought is was weird. I always thought people got higher velocities with SC. It has me scratching my head?? I don't see any apparent pressure signs. Primer is not flowing, etc.. Edited May 10, 2008 by Jack Suber Link to comment Share on other sites More sharing options...
XRe Posted May 10, 2008 Share Posted May 10, 2008 You didn't happen to switch lots of powder at the same time, did you? 8.8 sounds like a hottish batch - the slower batches are going to be around 9.1-9.3 for the same parameters. Primer flow is a hideous way to try to diagnose pressure issues I would stop and just double check everything - make sure nothing has changed on the press, etc, and that everything is the same w/ your chrono, too. Maybe take some of both loads out and chrono at the same time. I agree that that's a head scratcher... Smaller case volume w/ the same charge should result in higher pressure and therefore higher velocities, but.... Hmmmm..... If all else is OK, work up carefully and see what happens.... Link to comment Share on other sites More sharing options...
DrawandDuck Posted May 10, 2008 Share Posted May 10, 2008 I ran 7.8 gr of N350 which produced a 170 PF, swithed to supercomp, same load produces 169 PF, VERY little change. Good luck on figuring it out Randal Link to comment Share on other sites More sharing options...
dansy Posted May 10, 2008 Share Posted May 10, 2008 In my Dawson shorty with 2 holes 8.6 gr of 4756 make 171pf, in my previous batch I was only required 8.2gr to make the same PF. 4756 does vary a lot from lot to lot Link to comment Share on other sites More sharing options...
Jaxshooter Posted May 11, 2008 Share Posted May 11, 2008 If the gun is chambered for 38 super you may be loosing velocity from blow back around the case. The 38 super comp case is smaller in diameter than the 38 super. Link to comment Share on other sites More sharing options...
G-ManBart Posted May 11, 2008 Share Posted May 11, 2008 If the gun is chambered for 38 super you may be loosing velocity from blow back around the case. The 38 super comp case is smaller in diameter than the 38 super. Uh, if the case isn't sealing against the chamber wall, something is very, very wrong Link to comment Share on other sites More sharing options...
larry cazes Posted May 11, 2008 Share Posted May 11, 2008 (edited) Could it be due to differences in case neck tension from different case lengths run through the same crimp die setup? Edited May 11, 2008 by larry cazes Link to comment Share on other sites More sharing options...
XRe Posted May 11, 2008 Share Posted May 11, 2008 Could it be due to differences in case neck tension from different case lengths run through the same crimp die setup? That was one of the thoughts I had... It doesn't take a lot of difference in case length to change the crimp - although the velocity change that Jack is talking about is rather large for just lack of crimp causing the problem. Jaxshooter - you're thinking about rim diameter.... Link to comment Share on other sites More sharing options...
larry cazes Posted May 11, 2008 Share Posted May 11, 2008 (edited) Could it be due to differences in case neck tension from different case lengths run through the same crimp die setup? That was one of the thoughts I had... It doesn't take a lot of difference in case length to change the crimp - although the velocity change that Jack is talking about is rather large for just lack of crimp causing the problem. Jaxshooter - you're thinking about rim diameter.... Yup...but I have seen instances where velocity changes given changes to crimp only in a given load. I have my theories about this phenomenon but do not have access to the equipment needed to validate them. I would love to have access to a good ballistics lab for a few days..... Edited May 11, 2008 by larry cazes Link to comment Share on other sites More sharing options...
Jeff686 Posted May 11, 2008 Share Posted May 11, 2008 Did you chrony them on the same day with the same setup. I do a lot of experimental testing for my job, and I always include control samples. I attempt to hold as many variables as constant as possible, measure those not under my control, and vary only one on purpose. So, that means... load them on the same day, with the same lot of powder, same batch of bullets, same primers. Measure the OAL, crimp, case length. Chrony them side-by-side on the day/time. Send me the data, and I'll crunch it through a statistical analysis program. Link to comment Share on other sites More sharing options...
Flexmoney Posted May 11, 2008 Share Posted May 11, 2008 I'm with Dave, seems backwards. My first thought (like Larry) went to case-to-bullet tension (not merely crimp). Then, testing procedure, as Jeff686 mentioned. (hmmm....+1 to everybody. Which makes me wonder why I bothered to post. lol) Link to comment Share on other sites More sharing options...
Allgoodhits Posted May 11, 2008 Share Posted May 11, 2008 In addition to other checks, try measuring the case wall thickness of the other brass and the case wall thickness of SC brass. Just because brass outside dimension is supposed to be a constant, brass thickness does vary, thus changing case capacity, thus pressure variations too. MJ Link to comment Share on other sites More sharing options...
XRe Posted May 11, 2008 Share Posted May 11, 2008 SuperComp brass, in general, seems to be thicker than Super brass, but.... Link to comment Share on other sites More sharing options...
Jack Suber Posted May 11, 2008 Author Share Posted May 11, 2008 Follow-up - I found the problem.... When I tried the SC brass, all I did was load a few SC loads immediately after I loaded my match loads with regular super brass. Nothing changed on the press. They were chrono'd on the same day over the same chrono (Oehler). I assumed that the same charge was going into the SC case because I had not changed anything on the press. Well, that is not the case. I loaded some today, alternating cases between SC and Super. Turns out the the press throws a charge about .3 of a grain less in SC cases than the Super cases. So, was the SC brass hitting the powder measure funnel differently? I measured several pieces of the SC brass and compared to Super. SC brass is shorter than the Super brass. The new SC brass I measured averaged between .881 - 887" in length. The Super brass, both Starline and RP measured on an average of .891-.895". That has to be the reason. If the powder funnel is not being pushed up into the powder measure exactly the same, there should be variances in the powder charge. Wouldn't you agree? So, are chambers set up for SC shorter than those set up for Super? Link to comment Share on other sites More sharing options...
G-ManBart Posted May 11, 2008 Share Posted May 11, 2008 Follow-up - I found the problem....When I tried the SC brass, all I did was load a few SC loads immediately after I loaded my match loads with regular super brass. Nothing changed on the press. They were chrono'd on the same day over the same chrono (Oehler). I assumed that the same charge was going into the SC case because I had not changed anything on the press. Well, that is not the case. I loaded some today, alternating cases between SC and Super. Turns out the the press throws a charge about .3 of a grain less in SC cases than the Super cases. So, was the SC brass hitting the powder measure funnel differently? I measured several pieces of the SC brass and compared to Super. SC brass is shorter than the Super brass. The new SC brass I measured averaged between .881 - 887" in length. The Super brass, both Starline and RP measured on an average of .891-.895". That has to be the reason. If the powder funnel is not being pushed up into the powder measure exactly the same, there should be variances in the powder charge. Wouldn't you agree? So, are chambers set up for SC shorter than those set up for Super? The reason the new SC brass is shorter is that it hasn't been shot and reloaded a couple of times. Brass gets longer each time you shoot it (within reason). That's why rifle cases have to be trimmed back after being shot a couple of times or even once if they're on the higher end of pressures. I don't have the specs on hand, but I think other than rim diameter, the nominal dimesions of the two are the same. Link to comment Share on other sites More sharing options...
shred Posted May 11, 2008 Share Posted May 11, 2008 Follow-up - I found the problem....The reason the new SC brass is shorter is that it hasn't been shot and reloaded a couple of times. Brass gets longer each time you shoot it (within reason). That's why rifle cases have to be trimmed back after being shot a couple of times or even once if they're on the higher end of pressures. Actually that's because they're bottlenecked cases.. same thing happens with 9x25 and .357 Sig-- pulling the sizing ball out seems to be what stretches 'em. IME, straight-wall pistol cases never grow. Link to comment Share on other sites More sharing options...
G-ManBart Posted May 12, 2008 Share Posted May 12, 2008 (edited) Follow-up - I found the problem....The reason the new SC brass is shorter is that it hasn't been shot and reloaded a couple of times. Brass gets longer each time you shoot it (within reason). That's why rifle cases have to be trimmed back after being shot a couple of times or even once if they're on the higher end of pressures. Actually that's because they're bottlenecked cases.. same thing happens with 9x25 and .357 Sig-- pulling the sizing ball out seems to be what stretches 'em. IME, straight-wall pistol cases never grow. Bottlenecked cases certainly exhibit that to a much larger degree...no doubt, and it's a significant factor there, while it's not really in most pistol ammo. I just measured a handful of 1x and 3x fired Starline 38SC cases. They averaged .893". I then measured some multi-fired (5-6 times on average) Starline 38SC cases. These averaged .897". For comparison I measured new, unfired Starline 38SC cases and they all came in at .893 or 894....same as the 1-3x fired cases. To make sure I wasn't comparing apples to oranges I resized all of the above cases in an EGW/Lee U-die. That tells me that straight-wall pistol cases can and will grow slightly...at least under some circumstances. It's not a huge difference, but it could be just enough that the extra .004 is what gets the powder measure open that last little bit. Edit to add that I'm guessing that reloading the brass two or three times doesn't work the brass enough to the point that it stretches, but after it's been reloaded a few more times that changes. Just for fun I'm going to try and remember to measure the 3x batch after the next two or three uses to see exactly when they start to get longer. Edited May 12, 2008 by G-ManBart Link to comment Share on other sites More sharing options...
shred Posted May 12, 2008 Share Posted May 12, 2008 That tells me that straight-wall pistol cases can and will grow slightly...at least under some circumstances. It's not a huge difference, but it could be just enough that the extra .004 is what gets the powder measure open that last little bit. Wouldn't that just change the belling somewhat (assuming a Dillon-type setup with a suitable bell)? I've not measured my brass, I just know that >10x fired supercomp and TJ doesn't get too long. Link to comment Share on other sites More sharing options...
XRe Posted May 12, 2008 Share Posted May 12, 2008 My experience is that they tend to get shorter, but.... Actually, I'm going to empirically test some stuff shortly - and I'll be taking dimensions, so... Either way, Jack, yeah, that could make a difference in your powder charge. Re: how chambers are cut - my chamber runs either brass w/o issue, so.... They should be cut the same for both, but it doesn't surprise me that they appear to be different lengths.... Double check charges in the SC stuff, bump up if you have to, and see how it works out? Link to comment Share on other sites More sharing options...
G-ManBart Posted May 12, 2008 Share Posted May 12, 2008 That tells me that straight-wall pistol cases can and will grow slightly...at least under some circumstances. It's not a huge difference, but it could be just enough that the extra .004 is what gets the powder measure open that last little bit. Wouldn't that just change the belling somewhat (assuming a Dillon-type setup with a suitable bell)? I've not measured my brass, I just know that >10x fired supercomp and TJ doesn't get too long. Honestly, I'm not sure....I'd expect it would change the bell somewhat, but if the powder station die wasn't adjusted quite right, that difference could keep the measure from making a full throw. Strange no matter what the cause. R, Link to comment Share on other sites More sharing options...
G-ManBart Posted May 12, 2008 Share Posted May 12, 2008 My experience is that they tend to get shorter, but.... Before or after resizing? They definitely get shorter when you fire them, but resizing them should fix that. Look forward to your testing results. R, Link to comment Share on other sites More sharing options...
jostein jensen Posted May 12, 2008 Share Posted May 12, 2008 When we compress it to get thinner, I'd think it gets longer as well as reduce the inside volume. I'll try and measure a new one before and after size next time I reload, and do the same with the same case, once fired. Link to comment Share on other sites More sharing options...
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