- October 31, 2018 at 12:38 pm #68926Larry GibsonParticipant
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Pressure Question: GC vs PB original thread by TCLewis
A couple months ago TCLewis started this thread; http://castboolits.gunloads.com/showthread.php?367417-Pressure-Question-GC-PB where upon he asked the following;
“I am addressing this to Larry Gibson since he is the only one here that has pressure measuring equipment (that I know of).
Do you get any difference in pressure between gas checked and plain based boolits in the same loading?”
I responded with: “That’s a good question. I’ve not made that comparison. I could check my records but I hesitate to post any speculation based on comparing the pressure tests of different bullets probably conducted at different time/dates. The variables would be there and I’d get the usual raft of manure from the usual critics here.
However, if someone has a mould with different cavities that casts the same bullet for use with and without a GC I could conduct a test. All that’s needed would be a quantity of each cast of the same alloy. I would size, GC the ones for GCs and lube them for the test. Fifty of each would be sufficient for sighters/foulers and 3 ten shot test strings of each. Any takers?”
In short order Time Killer responded with this post; “ Larry I have one that I believe you are very familiar with the NOE 310-165-FN-H3 (30XCB). Its a 2 banger one gas check one plain base. If it will work how would you like them cast. I can do soft lead + tin for fill out, 50/50 soft lead / wheel weight + tin for fill out, or Wheel weight + tin if needed for fill out. Air cooled or water dropped let me know your preference. It will take me a few days to cast them as I have a busy work week but I would be happy to help out for the test.”
After a PM or two it was agreed that Time Killer would cast some of the XCBs, with and without G, and get them off to me. Time to for him to cast he bullets and mail to me, cogitating how to conduct the test, prepare the bullets and load them and a hunting trip (successful) have all passed and I am presenting the test results here. I decided to start a new thread as the other is already 3 pages long.
With the bullets Time Killer sent an explanation; he cast the bullets of COWW alloy + a little more than 2% tin added. The bullets were cast in an NOE 2 cavity mould for the 310-165-FN “XCB” bullet. One cavity gave bullets with a GC shank and the other was a PB XCB bullet. Other than the GC/PB the bullets were identical.
We assume a PB bullet of the same design will be heavier than the GC’d version because that’s what everyone has said for as long as I can remember. This is a prime example of why we should not assume as there is always some devil on the details. Picking 10 bullets of each style, GC and PB, and weighing them I found the GC’d bullets average weight was 163.7 gr. The PB’d XCBs averaged 166.4 gr. So far the assumption was correct……however, that is not the way we shoot them. We shoot them fully dressed.. both bullets mic’d out at .311 on the drive bands so I lubed both styles in a Lyman 450 using a .311 H&I and White Label 2500+ lube.. Hornady GCs were seated onto the GC XCBs. The fully dressed weight for the GC’d XCBs was 170.5 gr. The PB’d bullets, fully dressed, came in lighter at 168.4 gr.
I decided a fair, comprehensive and extensive test would be to test both bullets in 3 different cartridges, with 3 different rifles using 3 different burning rate powders (Unique, 2400, RL7) with equal loads of each powder in each cartridge. That amounted, using 10 shot test strings, to 180 test rounds plus foulers and sighters. I got everything together and waited for a front to move through for a decent test day. Yesterday I spent the day at the range and completed the testing. Here is the results.
Three cartridges were used; the 30-30 tested in a 21” Contender Carbine barrel, the 308W tested in a 24” barreled test rifle and the 30-06 tested in a similar 24” barreled test rifle. All 3 test barrels had a strain gauge affixed over the chamber at the SAAMI prescribed location for transducer psi measurement. The strain gauges are connected to the Oehler M43 PBL system which measures the psi. All 3 test rifles have been “calibrated” using factory “reference” ammunition. After setting up the M43 at the range it does it’s own “system check” and then was tested with my standard 170 gr load in the 30.30. The results of that test were consistent with what is expected of that ammunition so I began testing. The weather was fair with temps running 70 – 75 degrees, barometric pressure remained steady at 29.98 +/- and the humidity remained steady at 25% +/-.
Test ammunition; WLR primers were used throughout. The same lot of cases was used for each cartridge; FC cases in the 30-30, LC Match in the .308W and Winchester cases used in the 30-06. A Lyman .31 M-dies was used to expand the necks for the .311 sized bullets. No attempt was made to work up an “accuracy load”. Each load was selected based on velocity levels that are usually used for cast bullets of 165 – 170 gr respective of each powder. The bullets were seated to just touch the leade of each chamber. No wad or filler were used. Each cartridge was loaded into the rifle with the powder positioned at the rear of the case. With the Contender the barrel was pointed up as the action was closed. With the 308W and 30-06 bolt action test rifles the SAAMI “twist” loading technique was used.
The loads were;
Unique; 7 gr
2400: 14 gr
RL7; 19 gr
Unique; 10 gr
2400: 16 gr
RL7: 23 gr
Unique; 12 gr
RL7; 25 gr
I graphed out the psi results with a chart for each powder showing all 3 cartridge results. The GC’d XCBs are the blue column and the PB’d XCBs are the red column. The psi graph is charted on the left running from 0 psi to 32,000 psi. In the base of each column is the actual measured average psi. The blue column for the 30-30 on the Unique chart has 21.4 which means the measured average psi for the 7 gr load was 21,400 psi.
Velocities (the M43 gives a corrected velocity to the muzzle) in fps;
Unique; GC-1181, PB-1194
2400; GC-1688, PB-1701
RL7; GC-1768, PB-1732
Unique; GC-1384, PB-1432
2400; GC-1726, PB-1718
RL7; GC-1811, PB-1792
Unique; GC-1455, PB-1501
2400; GC-1709, PB-1691
RL7; GC-1815, PB-1777
Cartridge pressure ES/SD information listed by;
Powder; velocity – GC = ES/SD [psi =ES/SD] PB = ES/SD [psi =ES/SD]
Velocity is in fps.
A psi figure of “25” would be 2,500 psi.
Unique; GC = 43/13 [25/10] PB = 24/14 [5/3]
2400; GC = 57/17 [49/14] PB = 28/11 [52/21]
RL7; GC = 71/24 [71/21] PB = 94/38 [42/21]
Unique; GC = 47/13 [15/5] PB = 38/10 [15/6]
2400; GC = 27/9 [24/9] PB = 18/7 [9/4]
RL7; GC = 33/11 [36/12] PB = 29/11 [18/8]
Unique; GC = 54/20 [24/8] PB = 17/7 [14/6]
2400; GC = 14/4 [25/6] PB = 37/13 [21/9]
RL7; GC = 38/11 [44/15] PB = 24/9 [5/3]
As to accuracy; while I didn’t make any attempt to work or pick an accuracy load for any of the 3 rifles I did shoot for group at 100 yards. Any direct comparison of accuracy would be meaningless as such. However, suffice to say, each rifle performed well within it’s own accuracy capability. All 3 powders performed well with the GC XCB bullets. The .308W rifle gave the best group ever shot with cast in it with the 2400 load. All 9 shots tested went into a ragged 3/4″ hole.
Of the 30 XCB PB’d bullets only the Unique load in each cartridge gave any semblance of accuracy and then it was only fair in the 30-30 and poor in the 308W and 30-06. The 30 XCB PB’d bullet loads with 2400 and RL7 gave abysmal accuracy with few hits on the target……. judging by the cylinder bore size pattern impact around the target on the berm behind. Severe leading occurred with those loads also resulting in cleaning prior to the next test.
In comparing the psi differences I see very little. In 6 of the 9 tests the GC’d XCB gave slightly higher psi. In 3 of the 9 tests the PB’d XCB gave slightly higher psi. The psi variation in 8 of the tests falls well within the test to test variation that would be expected and the 9th test is border line within any test to test variation.
Based on these test results I see no reason to give any thought as to the use of a GC or not raising pressures. However, there still could be a difference based on similar weight bullets of different design based on bearing surface. That remains to be tested.
- October 31, 2018 at 1:40 pm #68929GoodsteelKeymaster
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Fantastic test Larry. I can’t say I’m surprised by the results, but a proven fact trumps a hazarded guess any old day of the week. Really excellent.
- November 3, 2018 at 7:46 pm #68945JniedbalskiParticipant
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