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Comparison using 30 vs. 40 degree shoulders
Alan Rushing
Member Posts: 8,805 ✭✭✭
With all else being equal ... what would the +s & -s with a given caliber / round?
I am aware of the gain in case capacity with increased shoulder angle but uncertain of how great or significant the differences might be between these two angles for a given round. How great an effect might there be with other factors (chamber pressure, throat wear, etc.?)
(If this is not the appropriate forum, let me know and I will throw it elsewhere ... thanks.)
I am aware of the gain in case capacity with increased shoulder angle but uncertain of how great or significant the differences might be between these two angles for a given round. How great an effect might there be with other factors (chamber pressure, throat wear, etc.?)
(If this is not the appropriate forum, let me know and I will throw it elsewhere ... thanks.)
Comments
When you see "results" exceeding this amount, you can bet your last dollar that the shooter has cranked the pressures way beyond what the parent cartridge was at.
Pressure is a function of capacity and powder charge. Increase the capacity with the same charge the pressure goes down. Increase the powder charge with the same capacity and the pressure goes up.
Throat wear has it's own set of factors, time between shots, powder burned/shot, total # of shots fired etc (rule of thumb for a 30cal is 40-42 pounds of powder before replacing the barrel)
At equal pressures, each 4% gain in case capacity will yield a 1% increase in velocity
When you see "results" exceeding this amount, you can bet your last dollar that the shooter has cranked the pressures way beyond what the parent cartridge was at.
Pressure is a function of capacity and powder charge. Increase the capacity with the same charge the pressure goes down. Increase the powder charge with the same capacity and the pressure goes up.
Throat wear has it's own set of factors, time between shots, powder burned/shot, total # of shots fired etc (rule of thumb for a 30cal is 40-42 pounds of powder before replacing the barrel)
so on average youll only get 6-7000 rds through a 30-06 befor the barrel is no good?
Maybe not good enough to hang in competition but good enough to keep shooting as long as it maintains the accuracy you expect from it.
Alan,
One of the things I've come up against in reloading 30 and 40 deg. shouldered cartridges is flow vs. pressure. You can build a lot of pressure by flattening out that shoulder angle. But you can also reduce the flow by doing so. That is why a lot of the more modern calibers have gone with shoulder angles closer to 30 deg. than 40 deg. The 6mm/6.5x47, The 6mmXC, the 6.5 Creedmoor, the WSSM's, the WSM's. They may not all be 30 deg. but most are closer to that than 40 deg.
That being said there are cases that prove their value with shoulders @ 40 deg. The .250 AI for one. It's about the same capacity as the 6/6.5mmx47 or 6mm XC except it's in it's original .257" caliber. It's a very efficient round that produces more velocity than the original .257 Roberts (a case size larger).
Anyhow, it's one of those things to be debated. Flow vs. pressure against the mindset of pure volume is what's making velocity. I happen to think this is also proven out by the 6mm Rem with it's 26 deg. shoulder. It's more efficient I believe than the .243 Win.
I get about 5000 rounds (I run 61gr of powder) from a 30-06 before the group average doubles in size.
A pure target shooter might dump it sooner, but their requirements are a little higher than mine.
IIRC that thumb rule came from the military
That 1/4 rule is proven; you get VERY little extra from an increase in case volume unless you load to significantly higher pressure. You'd see the same change if you simply loaded hotter in the original un-changed case. I am NOT advocating hot loads, mind you; I'm only pointing out that "improving" the case has almost no benefit.
For a guy with your background you floor me sometimes. On the inside of a turbine and even a piston engine 'impingement angles' in the airflow are VERY important. Why all of a sudden would they completely lose value here? I'm sure you and I could descend into a dissertation to each other, mechanic to pilot, about how stuff really works. But I don't agree with your blanket statement. I have a mechanical and an explosive ordnance background that say differently.
It's also a proven fact that of two cases with different volume and equal pressure when fired, the case with the larger volume is going to produce more velocity. {edit:}In some cases more a higher percentage, some a less higher percentage. I'm talking .257 to .257AI and .308 to 30-06. Both of the latter of each example produce more velocity. At the same pressure. And, I wouldn't call 100 fps VERY little. Also, if case shape means nothing, how come I can get better than published 25-06 velocities with about .257 Roberts loads ...with a 25 WSSM? Again, I think you need to take the blanket statement off and get to specifics.
One other thing improved cases do, in most cases, is give extended life to competition cartridges. Typically, loading up to higher levels, an average case can go 7-8 reloadings from a bolt action rifle. I've seen improved/modified competition cases go over 30 reloadings. That's also due to neck sizing in a tight chamber. I have TCU brass that has gone for 40 reloadings. And that's at max loads. So, once again, I believe there is a value to improved cases.
Bottom line: my problem with your statement is such that we might as well all just go back to the 30-06 and take what we get. I don't think that poo-poo'ing cartridge development, even if it only amounts to an improvement in marketing, is the right attitude about this topic.
There are 3 basic areas where you can increase capacity, body taper, shoulder angle and neck length (moving the shoulder forward)
Most cartridges designed post 1950 have already taken advantage of reducing the body taper and neck length.
In a couple cases, Ackley got a 20-25% increase in capacity (a couple of obsolete Winchester offerings) but most of the time he was lucky to gain 10% (and IMHO he wouldn't have bothered working on a "modern" case as not being worth the effort for perhaps a 3-4% capacity gain)
Compare the cartridges # 5 & 6 in photo 1 (30-30 Win)
to cases # 6, 7, & 8 in photo 2 (30-30AI, 30-30 Africian, 30 Aardvark, all based on the 30-30Win)
You can see the change in a rimless case (where they need the neck/shoulder junction to control initional headspace) below,
BTW the AI version has the same capacity and proformance as the 220 Swift
Yes, impingement angles can be important, especially in high gas flow situations. But cartridges have relatively low gas flow. There might be an argument affecting primer pressure wave "focusing" but the one cartridge I've worked with specifically designed to do that was statistically about the same as a cartridge with a standard shoulder.
The original post addressed ONLY shoulder angle, not shoulder location, body angles or case volume changes due to body shape. So I'll stick with my blanket statement that changing the shoulder angle will have a noticeable affect only on feeding. If you change a lot of things, you'd have more of an effect - but even then it will be only about 1/4 the percentage change.
Example: If you increase the case volume by eight percent (by any of the common means), you can expect only about a two percent change in velocity at the same pressure. If you had been getting 3000 fps, your "improved" round would burn a bit more powder to get 3060 fps.
Also, internally, the plasma exiting the sharper shoulder is more condensed in the centerline of the bore, and has been said(by some sources) to aid in reduced throat erosion (all things being equal)
I am a big fan of the AI 40* shoulder. Brass life alone makes the cost of conversion worth it.
Also, keep in mind, the heavily tapered rounds benefit by more than the 4% that most others gain. The 6.5x55, 257robts, 7x57, 220swift, etc all gain a lot from the minimal body taper, and increased internal capacity.