"Interchangeability" of load data question - DrMike

[sithlord6512]

DrMike (and others),

Before beginning please accept my apologies for the long post - could not make it shorter and still make it clear

I know - the common, "safe" recommendation in using load data is to only use the same combination of powders, primers, bullets, and brass as used in generating the data in the first place.

However, what if one knows that one bullet (180 g Partition) generates more pressures than another bullet (180 g Barnes MRX) with a given powder (i.e., Magpro) - can I "extrapolate" this information to other powders (e.g., H4350 data for a 180 g Partition will be safe for a 180 grain Barnes MRX because the Nosler bullet generates higher pressures than the Barnes bullet using Magpro)? If not, why not?

The reason I ask is that Ramshot and Accurate data (i.e., Western Powders) for 180 grain bullets in the .300 WM include data for four different bullets - (1) Barnes MRX, (2) Nosler BT, (3) Nosler Etip, and (4) Nos Partition. The maximum charges, velocities, and pressures using Magpro are as follows:
Max.chrg Pressure (psi) Velocity (fps)
180 Barnes MRX 85.5 g 62 398 3134
180 Nos BTip 85.5 g 63 390 3146
180 Nos Etip 82.8 g 61 891 3046
180 Nos Part 81.6 g 63 985 3085

The "relationships" presented above are the essentially maintained with Ramshot Magnum data.

I think that once you examine the data above, you will know what I am getting not. If not, please feel free to ask for more clarification

NOTE: I am not suggesting the relationships between bullets, velocities, and pressures extend to different bullets weights (e.g., Max charge for 150 NosPart in a .300 WM> Max Charge for 150 MRX in a .300 WM) or different calibers (e.g., Max charge for 180 NosPart in a 7mmREM MAG> Max charge for 180 MRX in a 7mm REM MAG - if there was such thing as a 180 g 7mm bullet). Instead I am wondering it this relationship would be true for all 180 grain .308 caliber rifles (e.g., 180 grain Nosler bullets WILL ALWAYS have lower Max charges than the 180 g MRX (or Higher pressures than 180 g MRXs for a given powder charge).

 

[Guest]

I'll try to take a short stab at this, sithlord. In a nutshell, you are surmising correctly that, on average a given bullet will generate pressures specifically relative to a different bullet over the same powder and in the same case, with some consistency. Key words here are "on average," "same powder," "same case," and "some consistency." Is it safe to say that the 180gr E-Tip will generate more pressure per grain of powder X, in cartridge case Y, than the 180gr AccuBond over the same powder in the same cartridge case, if both are loaded and shot in the same rifle, and with all other variables such as lot numbers for primer and powder and brass, being equal? Yes. But, what does that tell you? It tells you E-Tips produce higher pressures. It tells you that if you have a safe E-Tip load of XX.Xgr of Powder X, you'll probably be safe when you work up to that with the AccuBond of the same weight, in the same case, with the same powder, and the same lots of components, in the same rifle. But you should still work up carefully, as there are anomalies possible and you don't want to put loads in a rifle "at risk" of making bad assumptions.

If you're asking whether or not it's generally considered safe to start with the start charge from load data using a different bullet of the same weight or a different primer or brass, it is generally considered safe to do so, and to work up carefully. But, the key is understanding the issues you're already looking at. And it appears you are making some wise notes as to how these things relate. Safety is the primary concern, and it beats the heck out of 100fps of extra velocity, every time.

In some instances you won't have data for the exact combination of components you have available, in which case it's alright to extrapolate from known data as long as you understand what's going on and how it all relates. For example, I have a Remington 700 Classic in 8x57, and I wanted to shoot 185gr Remington Core-Lokt bullets in it. No data exists for these bullets. I can find 180gr and 195gr data, but nothing for 185gr, and nothing for these particular bullets. So I went through load workups very carefully, and found loads that worked for these bullets. I've since switched to the Nosler 180gr Ballistic Tip, because I tried them, too, and found them more accurate, but initially I worked up off of extrapolated data and I was very careful while doing it.

I hope that answered your question.

 

[Antelope_Sniper]

Sithlord.
When ever I evaluate issue of this nature I look at the totality of the evidence. I know that certain bullets, brass, and powders typically produce less pressure then other. I also know some rifles are more forgiving in this area as well. Seldom are my recipe's an exact duplicte of what's in the book. If a book lists on 6s ogive cup and core bullets of a given weight, I'm pretty comfortable using that data for any other 6s ogvie cup and core bullet. Of course I would expect a similar construction 4s to produce more pressure, and adjust my start point accordingly. I load alot of non-canister powder, and sometimes only have a good guess at what the burn rate might be. This has kind of lead to my Motto. Start low, and when in doubt, START LOWER, then work up. This saved my bacon once. I was loading a strange powder for my .338. I thought I know what a good starting and end point should be, but decided to start lower, way lower, just in case. What I initially thought would be a good starting load was actually over max. But since I started low enough, I stopped before I ever reached them, and pulled the bullets. In the end I got a good new load that shoots one hole, and a good reminder how discresion is the better part of valor.

On the other hand I have "max" loads I've shot in a dozen plus different rifles, and I've never had a problem. I know these loads well enough I'm comfortable shooting them in any qualtiy built, modern bolt action rifle in good repair.

 

[sithlord6512]

Thanks guys - you have provided the kind of answers I was hoping for. My plan is to use the "safest" data possible when developing loads for a powder + bullet combo for which I have no published data for. For example, If I want to develop a load using IMR4831 for a 180 grain Barnes MRX (a powder not listed in the Barnes #4 manual or Hodgdon website), I know that I "should" be safe starting at the min load listed for 180 grain Nosler bullets in the Nosler Manual #6 for this powder and working up to the Maximum velocity listed in the Nosler book (of course, assuming no other pressure signs are indicated before reaching this max).

Reason: Ramshot and Accurate data indicates the Max charge for 180 g Nosler Partitions is lower than the max charge for 180 g Barnes MRX's (i.e., the max charges for NosPart should be lower than Max Charge for MRXs using the same powder) AND that the Max velocity at the max charge for 180 g Partitions is lower than the Max velocity at the max charge for the 180 g MRX (i.e., stopping at the max velocity listed for a 180 g NosPart should be safe when using a 180 g MRX - assuming overpressure indicators not observed before then).

While this practice would not allow one to reach the maximum potential of the bullet/powder combo in question (e.g., the 180 g MRX could probably pushed to higher charges, higher velocities), at least one could be reasonably confident that the load is safe.

Thanks again.

 

[ScreaminEagle]

I see what you are getting at. I have used load data for one particular bullet and just used a different bullet. I will never substitute powder but have used different cases, bullets, and primers, though I always drop down a couple grains and work my way up just to be safe. What you are doing is fine.

 

[DrMike]

sithlord,

You got some pretty good advice. The short answer to your question is that you cannot precisely extrapolate between loads built with different bullets. I state the answer in that way because the relationship between various loads depending upon differing bullets is not linear due to differences in metallurgy, differences in bearing surfaces, and even differences in rifles used. Also, remember that generation of the pressure curve is exponential rather than linear. If a load is apparently safe with a similar weight bullet, and the charge does not differ significantly, drop back to the minimum and work up carefully, watching for pressure signs. I'm rushed today, but I'll give this more attention by tomorrow. On a full unknown condition such as you suggest, a 10% reduction in charge may well be warranted.

 

[Greg Nolan]

Ditto to all.
It helps to have a lot of load data by different bullet and powder companies.
Also, as DrMike mentioned, there are exponential rates and this can be a double edged sword in that the pressure curve may be sharp enough that too low a starting load may not have sufficient initial pressure to start the bullet into the rifling forcefully enough thereby causing a temporary blockage that can cause a catastrophic spike in pressure.
Greg

 

[sithlord6512]

the relationship between various loads depending upon differing bullets is not linear due to differences in metallurgy, differences in bearing surfaces, and even differences in rifles used.

I guess I am making a couple of potentially dangerous assumptions with respect to the Ramshot (Magnum) and Accurate (Magpro) data for 180 g MRX, Btip, Etip, and Partitions:

I am assuming that
a) the same .300 WM was used in generating the data for these four bullets (180 MRX, Btips, Etip, and Partition)
b) the same lots of Magnum and Magpro were used in generating the data for these 4 bullets
c) there is no bullet/powder "interaction" (e.g., All other things being equal, the relative relationships in max charges,velocities, and pressures between MRX, Btips, Etips, and Partitions would be maintained, regardless of the powder used - e.g., the Max charge and Max velocities of .308 cal., 180 g Partitions would always be higher than for the MRXs). That said, I can't imagine how a bullet/powder interaction would work (e.g., how changing the powder used could cause a reversal such that the 180g MRX max charge and velocity would suddenly be lower than for the 180 g Partition).

Any deviation from these assumptions would invalidate my premise that the Magnum and Magpro data establish a useful, "relative" ordering of the charge/pressure characteristics of these four bullets (particularly for the MRX and Partition - opposite ends of the "spectrum" illustrated by the data in my initial post)..