Why the tremendous difference in numbers

[mugsie]

Fist some background. I usually shoot 168g SMKs out of my .308, but managed to score some 155g Hornady A-Max's. So I started working up loads, referring to manuals, sites etc.

The latest Hornady reloading manual lists:
Varget 35.9 - 44.9 grains IMR 4064 38.4 - 44.9 grains

The Hodgdon site lists:
Varget 44 - 47 grains IMR 4064 43 - 47.5

These are tremendous differences! The Hodgdon site starts Varget .9 grains less than the max load in the Hornady manual and IMR4064 1.9 grains less than the Hornady max.

I will be working up in .4 grain increments, from 41 grains using IMR4064, but this is an arbitrary starting point. I could have started at the low end of the Hornady manual up to the high end of the Hodgdon site, but the span would have been far too wide. Other than the Lawyer factor, why does one think these weights vary so much? One uses a barrel while the other uses a copper crusher? Even if this were the case, I would think they would be relatively close. Pressure is pressure.

My Speer Manual also has charge weights in line with the Hodgdon site, however these are for SMK HPBT bullets, whereas the Hornady is an A-Max. It's a BT but not a HP. I also called Hornady and they said the bearing surface of the A-max was greater than the SMK, but again, it seems like far too wide a difference.

Any thoughts? Anyone have experience with 155g A-Maxs and these two powders? I'd be interested in hearing all of your rational.

Thanks people, and stay safe....

 

[Trek]

You are asking a question I have been pondering for years. This can be very frustrating. The best I can figure out is that each combination of Rifle/barrel, bullet, powder lot number, primer, and case is unique, and no two combinations will give the same pressure for the same weight of powder and bullet. Bullet design can have a tremendous difference. I have seen cases where just changing the bullet shape/design can be the difference between a very safe load and a very dangerous overpressure load. The bearing surface can have a tremendous affect on the resistance of the bullet to start as well as during its travel down the barrel. The construction of the bullet can make a huge difference. Solid copper or Gilding metal bullets often have a very high resistance in the barrel compared to regular jacketed bullets. Unless we have pressure testing equipment, we cannot always predict what a change in just one component will make to the pressure curve.

What I find more frustrating is that I do not recall every reading how to handle this situation in any of the reloading manuals. Of course, some of them will say to not substitute different components for the ones used for the load data. That is often easier said than done. What if the bullet is no longer available, or maybe it has been back ordered for a year everywhere we look. What do we do then?

 

[mugsie]

Trek - You're right of course - but this is one hell of a wide difference in powder loads. Now I already know not to go near the top end of the Hodgdon numbers because of prior experience with high pressure signs. One pierced primer was enough. The Hodgdon site is right on the money for the SMKs, but too much oomph for the A-Max's. That's why I'm starting low, or relatively so, in the middle of the range and working up. If that doesn't work, then I'll work down. It's just frustrating to see so much variation in powder weights. I guess that's what makes it fun huh?

 

[gamestalker]

I've been dealing with this for years, as I'm sure everyone else has.

What I do, is I take the various sources of data, add them together, divide by the number of sources to obtain an average start and max charge. This approach has never gotten me in trouble when working with bottle neck cartridges.

For example, lets say you have 3 sources, one has a start charge of 34.0, another shows 35.5, and yet another shows 33.4 grains. I add all three together, then divide by 3 to get an average start charge of 34.3 grs.. Then do the same thing for the max charge, and you'll have a more reasonable table to work up with. Not too high, not too low.

And of course, always take bullet profile into consideration. Some bullets use a different charge table because they either have more or less bearing surface, or the construction may be drastically different than a typical cup & core spitzer SP, or what ever, in other words, not all bullets are created equal.

GS

 

[Bart B.]

People all hold rifles different amounts against their shoulders when chronographing a given load and rifle. That alone, will easily show up to 100 fps spread in average muzzle velocity and spreads in standard deviation.

If that same barreled action is hard mounted like a SAAMI spec test barrel in a universal receiver so it doesn't move back in recoil when fired, that load will have a higher muzzle velocity and lower standard deviations than anyone's data hand holding the rifle. This is the primary reason most people get lower fps numbers with factory ammo that their specs state.

Every load developed without SAAMI spec mounted barrels and quality piezo transducers for pressure and velocity is measured with a rubber ruler. A poor quality one, too. Which is why some book data is over SAAMI pressure specs.

 

[spitballer]

I consider reloading data as guidelines, although I learned my lesson and I never start at the upper range. Seems to me what we all want is accurate groups, and this means accuracy beyond the first shot. Accuracy beyond the first shot means laying down proper fouling on the first shot, and I'm convinced that the charge necessary to accomplish this is barrel-specific and can't be established for everyone with a chart, although I believe that for most folks it will be at the low end, possibly even lower for long guns. Carbines and FMJ's I'm not too familiar with.

 

[Bart B.]

I'm convinced some loads shoot very accurate across all sorts of barrels as long as they're installed properly and have internal dimensions at or close to SAAMI specs. Why else would Federal Gold Medal Match as well as Hornady and Black Hills match ammo do that?

Especially when several of us developed a load for a new Sierra 30 caliber match bullet, then loaded several thousand rounds and it all shot about 1/2 MOA at 600 yards across 20 to 30 different barrels with varying internal dimensions.

I'm also convinced if 20 reloaders picked at random were given their choice of components to put under a given bullet, there would be 20 different component types and amounts they thought was most accurate.

 

[spitballer]

Published load data appears to be based on the first shot out of a cold barrel, and in my experience first shots are generally predictable and accurate as long as they are within established guidelines. But the subsequent shots may not be. Just why seems to remain a mystery, but personally I suspect it's a matter of laying down proper fouling, and I've had very good results when I load rounds to a specific velocity that my barrel "likes" and let the powder speed determine where it tops out. Again this is for soft varmint bullets, I don't pretend to know about ball ammo.

 

[Nature Boy]

If you're trying to load the Hornady 155gr AMAX, why not just stick to the Hornady book?

The Hodgdon website load data is for a generic 155gr bullet. I assume that because no other info is given.

BTW, I love the 155gr AMAX with 38.6gr of H4895. It allows me to consistently shoot between 1 - .075 inch 5 shot groups at 100 yards with my Springfield M1A. I'm my short reloading career, It's become my go-to round

Comparison between the 155gr AMAX and 168gr SMK (for me and my rifle)

 

[edfardos]

They differ by 5% or so in your example. If you want to see the error in their process checkout the tests between win231 and hp38. Its the same stuff, but a single tester will list different max loads. I cringe a bit when people say only use published data and you're perfectly safe. I say, use the average of all published data, then also get anecdotal evidence here if you can.

Edfardos

 

[Trek]

I agree whith you edfardos that people should not say you are perfectly safe if you follow published data. I do not remember the publisher of any reloading manual saying that. I think that people get the wrong idea of exactly what is published in those manuals. I certainly do not interpret the loading manuals that way. One time the tech support of one of the well respected reloading manuals (I forget which one) told me that the proper way to use a reloading manual is to consider it a report of test results, conducted at a particular time with a certain specific set of conditions, and not a "cast-in-stone, you will get exactly the same results" type of commandment. He emphasized that this was a report of a particular test with specific conditions, such as a particular test barrel, at a certain temperature, atmospheric pressure and humidity, with specific brands and types of components of a specific, unique lot number (cases, primers, powder, bullets), on a particular day. If you change any of these components, you should expect different results. If you use the same brand and type of components (but of course most likely different lot numbers, since they do not publish the lot numbers they use), in the properly chambered firearms in good condition, the published START LOADS are most likely safe. There is however no guarantee that the published MAXIMUM loads will be safe for your rifle and your unique set of components and conditions.

There are seldom any 100% guarantees on any endeavor, and each activity has its unique set of risks and rewards. Even shooting factory ammo involves some risk that it could damage your gun, or worse. I feel that the risks posed by hand loading are worth the rewards.

 

[spitballer]

I'm inclined to agree with edfardos too, anecdotal evidence regarding twist rate could be important.

 

[mugsie]

Well I went to the range today. I picked an arbitrary start number and worked up towards max. I guess my experience accounts for something because my first grouping was right on the numbers. I started .010 off the lands for all groups. Now I'll work on seating depth. All shots below are three shot groups except for the yellow which were 168g fowlers. The others are IMR 4064 with 155g Amax's.

 

[mugsie]

Oops, here's the image.

 

[bfoosh006]

There are far more variables then just charge weight at work here....

Some of the possible differences...

Case capacity
primer used,
test barrel used,
test barrels chamber design and leade length and throat,
test barrel's actual bore size and land size and type,
different Lot #'s,of powder,
bullet overall seated depth,
type of bullet used....

While that is not a direct answer for you... do bear in mind those all can drastically change a loads max charge weight.

As food for thought... read this.. it is an example of how drastic even a simple .357 Mag. load can be in various revolvers.

Pay close attention to "S&W M19 #3" and "Colt Python #1"..... by all rudimentary thought .. those should be much more similar to the identical type of gun..


http://www.leverguns.com/articles/ballisticians.htm