Increased bulletproof weight, gas checks, and quantity of smokeless powder

[orpington]

My understanding is that increased bullet weight increases pressure. My understanding is that gas checks increase pressure as well. So, why would the published powder load, in this case IMR 4227, be greater for a cast bullet with a gas check vs its counterpart, it also being cast but of lesser weight and with no gas check?

 

[Jim Watson]

Please be specific and complete not general and hypothetical.

 

[243winxb]

, why would the published powder load be greater for a cast bullet with a gas check

The gas check provides protection of the bullets base & can be loaded to higher pressures.

 

[res45]

My understanding is that increased bullet weight increases pressure. My understanding is that gas checks increase pressure as well. So, why would the published powder load, in this case IMR 4227, be greater for a cast bullet with a gas check vs its counterpart, it also being cast but of lesser weight and with no gas check?

Friction.

 

[Barbaroja]

Takes more pressure for a jacketed bullet to reach a certain velocity than it does a cast bullet due to increased friction. I’d venture a guess that the gas check increases friction a bit over a plain base lead bullet. Also as 243winbx pointed out above me, You can usually push gas checked Bullets faster than plain base before problems arise.

 

[jmorris]

How much volume is left in the case, if the lighter bullet takes up more case volume, that would be one good reason.

If the heavy bullet is going the same speed or slower than the lighter bullet, that too would make sense.

Of course if the data is from more than one source, that’s almost a guarantee they won’t be the same.

I have some books that lists different starting/max loads for the same bullet, seated to the same depth using the same powder. This H110/W296 (same powder) data as an example.



Different lot of powder, different environmental conditions on the days tested, different measuring equipment…

 

[murf]

copper melting point is much higher than lead. more powder means more heat/melting of lead (not copper). my lead 158 grain 38 caliber bullets start melting @ 1250 fps which is a lot less velocity than a max load. with a gas check i could push that bullet another 200 fps.

murf

 

[Charlie98]

My understanding is that increased bullet weight increases pressure. My understanding is that gas checks increase pressure as well. So, why would the published powder load, in this case IMR 4227, be greater for a cast bullet with a gas check vs its counterpart, it also being cast but of lesser weight and with no gas check?

To sum up your questions...

Increased bullet weight does, indeed, increase pressure. That's why you will see load charges go down for a heavier bullet, when comparing apples to apples... that is, say a Hornady jacketed bullet vs a Hornady jacketed bullet.

Gas checks seal the base of the bullet, much like a jacketed bullet obturates... increasing pressure with a better seal. The benefits of a gas check are two-fold... besides a better seal, you can generally drive gas checked bullets for this reason, and as was mentioned, reduce leading from a hot flame front.

Cast bullets can run from very soft (pure lead) to very hard (typically called 'hard cast.') There are benefits to both... depending on what your purpose is for the bullet, and how fast you plan on driving them. Again... generally speaking, soft lead bullets cannot be driven over the generic threshold of 1200fps (or even less...) without leading and other problems. Harder cast bullets can... I regularly drive non-gas checked cast bullets into the 1800's, and gas checked cast over 2200fps.

There are also exceptions... like the .45 Colt. A generic .45 Colt load won't drive any bullet over 900fps (or thereabouts, depending on the weight) but there are firearms that can handle heavier loads, and data is provided for those specific firearms... using the very same bullet in some instances.

 

[orpington]

Please be specific and complete not general and hypothetical.

I believe I was comparing data from different publications and hence my confusion. Further research reveals that these publications consistently appear to decrease powder suggested starting grains and max loads as bullet weight increases. However, there is variation using the same bullet weight and a different publication.

 

[AJC1]

Velosity limits sometimes limits charge. You can see this with 4227 in 308. Its never a pressure problem, understanding the limiting factor can explain the why. Gas checking the same bullet can increase speed tollerance, weight of bullet and charge all at the same time.

 

[41 Mag]

Different lot of powder, different environmental conditions on the days tested, different measuring equipment…

As was pointed out by jmorris and Charlie, if all things were equal, but they're not. Even barrels made connectivity will produce differences.

The books are guides, not absolutes. That is why we start low and work up, verses picking a weight and going for it. Works the same with lead, my alloy and cast 432-640 bullet might be softer than yours or either might be a plain base or GC'ed. Either way the only data might be a Lyman manual. That said, for a 260gr cast bullet with a similar profile across the board "should" have similar start load data.

Will your alloy and firearm be the same as ours or theirs, nope so we work with what we have recipes for.

 

[Charlie98]

However, there is variation using the same bullet weight and a different publication.

A few other things to look at... are each data sets finer details. Things like which primer, is it a test firearm or a test barrel (2 VERY different things!) and the length of the barrel. As .41 suggests... every set of load data you see is really valid, just valid for what the tester that day found using their components in their particular environment. Testers also have different end goals for data... accuracy vs velocity, or pressure over burn characteristic... even something as esoteric as case fill.