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Post by rbinar on Feb 28, 2009 21:01:27 GMT -5
It’s time to rediscover some things that happened a few years back. The images and thoughts they generated provide insight to 50 caliber loads even today. This post is for those unfamiliar with the data but those who were there might want to comment as well.   The first two images are book loads. The first is 44 grains of N110 under a 300 grain bullet and sabot. The second is the same weight of Xmr-5744 under the same 300 grain bullet. The two traces provide a guideline for the maximum pressure I thought should be shot with a sabot in 50 (or any other) caliber. There is nothing particularly startling about the traces. What might be a surprise is 40,00psi can be reached and sabots survive. At one time that was considered beyond a sabot but was soon dismissed. Another lesson was that factory loads with a 300 grain bullet shoot to considerable pressure and that pressure was necessary for load insensitivity.  68 grains of H4198 under a 300 grain bullet.  74 grains of H4198 under a 250 grain bullet The next two images were a surprise to many. They show that even a large amount of a slower burning powder would shoot to below maximum factory load pressure. Neither load makes 40,000psi yet both shoot the described bullet to well beyond factory speed. The loads also show why it is necessary not to shoot too LITTLE a charge. If much less powder is shot then there’s the real danger of pressure falling below the mid 30kpsi range. At those low levels it’s possible to introduce considerable temperature effect.  The last image shows to what extent one may have to go to achieve reasonable pressure. N130 is a favorite of mine but it takes a lot to reach pressure with a 250 grain bullet. This 82 grain (yes I said 82) load does not enter a maximum pressure area despite the power generated. The recoil produced from a full pressure load (85 to 87 grains) is immense. That means N130 and like powder 10X are too slow for most shooters using 250 grain bullets. They are best left to 300 grain and heavier bullets There are a number of things that were learned by the traces and other pressure measurements. It was one of my most fun projects ever. They can still provide enlightenment to those wanting to understand the role of pressure in how a rifle shoots. |
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Post by DBinNY on Feb 28, 2009 21:09:32 GMT -5
RB, thank you. Your traces have provided a lot of enlightenment to me. You are, without a doubt, the reason I use R7 with 300 gr. bullets. Any idea why that first trace is bimodal?
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Post by Dave W on Feb 28, 2009 21:21:51 GMT -5
I can't tell you how many times I have looked at those traces on the old board, great stuff!
Did you do any traces for .45's RB?
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Post by younghunter86 on Feb 28, 2009 21:59:14 GMT -5
rb-
Thanks for revisiting. The grapics are great. It is very good way to help newbies like me understand these things. Now that I've seen this, I have more random thought running through my head. What would a curve look like for a duplex? Does it essentially reach max pressure quickly and then sustain it? Hence the best of both worlds?
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Post by rbinar on Feb 28, 2009 22:59:21 GMT -5
RB, thank you. Your traces have provided a lot of enlightenment to me. You are, without a doubt, the reason I use R7 with 300 gr. bullets. Any idea why that first trace is bimodal? Two possibilities: Slight interuption to the pressure transducer caused by it getting hit by the cable, my hand...ect. during recoil. Inertial resistance of the bullet changing during acceleration: sabot loosening, barrel oil or anything that could cause slight loss of friction in barrel. Ockham's razor favors number 1 |
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Post by rbinar on Feb 28, 2009 22:59:51 GMT -5
I can't tell you how many times I have looked at those traces on the old board, great stuff! Did you do any traces for .45's RB? Not YET. |
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Post by rbinar on Feb 28, 2009 23:02:19 GMT -5
rb- Thanks for revisiting. The grapics are great. It is very good way to help newbies like me understand these things. Now that I've seen this, I have more random thought running through my head. What would a curve look like for a duplex? Does it essentially reach max pressure quickly and then sustain it? Hence the best of both worlds? Actually a duplex is not much different from a normal load except the curve is rounder and less a spike. Something like this. i8.photobucket.com/albums/a15/dougva34/11-N110X2064-H322.gif |
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Post by dougedwards on Mar 1, 2009 8:55:30 GMT -5
I have seen these graphs for years and it appears that pressures are on the low side for temp insensitive and reliable ignition. Maybe anything in excess of 42,000 psi or so is too hard on the sabot. I don't know. But when no sabot is required I would think that pressures in excess of 44,000 psi would be favorable for reliable ignition and for proper bullet obturation with some projectiles. I am way off base here?
Doug
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Post by jeff on Mar 1, 2009 10:11:09 GMT -5
Good stuff!
Thanks, Jeff~
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Post by sw on Mar 1, 2009 10:23:22 GMT -5
I have seen these graphs for years and it appears that pressures are on the low side for temp insensitive and reliable ignition. Maybe anything in excess of 42,000 psi or so is too hard on the sabot. I don't know. But when no sabot is required I would think that pressures in excess of 44,000 psi would be favorable for reliable ignition and for proper bullet obturation with some projectiles. I am way off base here? Doug This line of thinking was why I went with such an extreme loading for my 40 cal this year. Wanting serious temp insensitivity, I went with the 11/46 duplex of 4759/2015/200SST/2670'/sec which likely slightly exceeded 50K. It is very temp insensitive(<50'/sec decrease with 60 degree temp decrease) but this is too extreme IMO. My loads for both 45 and 40 have the goal of 44-45K. This is to compromise between ventliner wear rate and temp sensitivity. Actually, 50K is well under 1/2 of what the gun should easily take. RB's BPs are very robust, still working well south of that has it's merits. |
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Post by sw on Mar 1, 2009 10:28:17 GMT -5
:)RB's Pressure Trace readings are very helpful. My thought is that the quicker the initial presure rise, the more consistent the velocity will be - all else equal; and the slower the pressure bleed-off, the faster the load will be. Duplexing helps on both of these. This is my reasoning for duplex.
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Post by bigmoose on Mar 1, 2009 10:50:02 GMT -5
Rick,
Very interesting, I really beleive you don't fix what AIN"T broke, so all my shooting, is done with the loads you sent me.
I leave experimenting to others, and with results I am getting, I would be crazy to change.
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Post by rbinar on Mar 1, 2009 12:22:43 GMT -5
I have seen these graphs for years and it appears that pressures are on the low side for temp insensitive and reliable ignition. Maybe anything in excess of 42,000 psi or so is too hard on the sabot. I don't know. But when no sabot is required I would think that pressures in excess of 44,000 psi would be favorable for reliable ignition and for proper bullet obturation with some projectiles. I am way off base here? Doug Doug your idea is right on. It is also the reason for this post. It seems that there are many who have great interest and knowledge of ballistics after the bullet has left the barrel. The number who understand or show interest in what happens inside the barrel is much less. That may be because many see the interior workings so much smoke and mirrors. It's not that difficult with some time and help. Bench shooters were mentioned in the beginning because they shoot what many would think of as extra high pressure. Pressure has benefits. A load that shoots a required bullet speed at the lowest pressure possible is not necessarily a good idea. Stability can often be a problem. Higher pressure is more stable in almost every case. But high pressure has problems as well. So how do we determine what pressure is best in a given application? It is best done by having a clear idea what you wish to do at the start. If you are a long range paper shooter at least a nominally high pressure is required. The simple reason is efficiency: when more powder is turned to gas the variation between shots is less. More powder is burned at pressure so there is an automatic return for more efficient (higher pressure) loads. A bench rest shooter has things to do to get efficiency that a hunter would consider no return in the field. First off a bench rest shooter would have his powder chamber COMPLETELY polished and chromed. This would help reduce erosion and micro fracture. Also vents would be changed an average of 10 shots maybe 5. Parts (and some would say money) don't mean anything to a bench shooter. So we can't ask a hunter to take on bench rest pressures. Then the right pressure is a compromise. Performance is weighed against cost and ease of use. Some would say safety belongs in the compromise, but I'd say don't even attempt a load that can't be totally safe from the beginning. The limits I've listed make what may be good sense to some and lacking to others. If you know what you're doing and higher pressure is your side of the compromise that's fine by me. The 40,000psi sabot limit has been known for years. The less than 50,000psi sabot-less limit is chosen with the ideas you mention. I built a natural protection in the idea by stating that about 45,000psi was the target and I mostly restrict what I recommend to that because it's where the compromise of efficiency and practice are best for me. |
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