What is the general effect of loading lighter bullets using data for heavier bullets. IE: loading 200 gr. with data for 250 gr. bullets?
Does the pressure decrease with a lighter bullet and increase with a heavier one? How about bullet velocity? Would lighter bullets run faster? I'm assuming the bullets are not greatly different in length.

Thanks

Sometimes your rifle blows up. Bullet loading data is set up to match powder volumes with bullet weight. What your suggesting is extremely dangerous. You need to get several reloading manuals and read them from cover to cover. This kind of stuff is discussed in every loading manual.

Ok, let me clarify. I was thinking about revolvers; specifically I was looking at 230 gr. data to load 200 gr. bullets using black powder. THIS IS A CONCEPT QUESTION ONLY. Believe me, I follow published data very carefully. However, I'm just unclear on the concept of the effects of heavier or lighter bullets. I know that longer bullets can cause excess pressure because they are generally loaded deeper. I was just trying to get INFO ONLY on similar wt. but lighter bullets.

Black powder is a whole different ball game. No matter the bullet weight, you want the case full, and a little compression is generally good. Not filling the case with black powder is considered a bad thing.

Smokeless - pressures will be lower. How much? Impossible to say for sure without a whole lot of specific information. It could be OK and it could be downright dangerous. Again, you'd have to give a very specific example.

If you'd really like to satisfy your curiousity, a program like Load from a Disk can suggest what will happen.

Thanks for the "Load from a Disk" info. The demo looks pretty useful.

Stay with the concept and then let it die! I would never use data for a bullet different by more than 10 grains. And who knows what the changes are for blackpowder? It sure isn't a straight line of, 10% less bullet weight, so 10% less powder. The physics just don't work like that.

Talking smokeless powder, some powders are not recommended with lighter bullets. If the propellant is appropriate: for a 10% lighter bullet I'd expect about 4% increase in velocity and about 8% decrease in pressure for the charge as long as the cartridge case volume is not changed (as in the bullet is the same size but lighter due to a change in alloy density.) If the bullet is smaller and COL is maintained the following should be of interest: for a 10% increase in cartridge case volume I'd expect about 3% decrease in velocity and about 13% decrease in pressure.
A 200 grain bullet is 13% lighter than a 230 grain bullet and 20% lighter than a 250 grain bullet. Is it safe to extrapolate that much from the 10% change in bullet weight I mentioned? Would reality conform to my stated expectations or would things get erratic? Are you changing bullet type as well as weight? I have more questions than answers.

A few years ago I was active in Highpower competition. I was often looking for quicker ways to make loads so I chose a powder charge that worked well with the heavier bullet I fired at 600 yards (42.5gr Vit N140, 175 gr Sierra Match King in 308 Winchester) Velocity averaged 2519 fps. I then seated a 168 on the same charge and obtained 2494 avg, A 150 seated on top was 2547 fps. Now with this powder all three projectiles could easily have been given better performance but all three loads worked extremely well for each yardline fired in highpower matches. It is interesting that the velocities are similar. I looked in the Lyman 48th edition load manual at loads for the 44 Magnum. Using Alliant 2400 and a 300 gr. bullet max charge of 17.5gr, 934 fps at 37,800 cups was obtained in Lyman's test. A 240 gr with 17.5 gr will fall much lower than the max listed for that bullet in which 20.8 grsaw 1154 fps at 37,700 cup. A 200 grwhich has a max listing of 23.6 gr gave 1227 at 35000 cup. Now if I load at 17.5 gr I can see that I would be quite below the listed starting load of 19.5 gr to get 972 at 23000 cups. The problem here might be that I could see the problem I have heared about of having excessive airspace in this volume of case causing "detonation" symptoms. Now I admit In this area I do not have knowledge but in the rifle case the above mentioned charge and the different weiht bullets and length of bullets were not very different to be un safe. They seem to stay in the parameters of minimum and maximum charges. I think I would use more caution in regards to straight walled handgun cases especially of high volume when the data is for bullets that vary in wieight so far apart. If the difference in weight is moderate I would have no concerns to work up a load to the similar charge I am interested in for a heavier wieight bullet.

Let it go, Cukrus...the chances of something very bad happening are quite high. Use the published materials out there, there's certainly NO lack of info these days: The bullet company sites, the powder company sites, these pages, and Steve's pages...and you'll have far more info than you can use. No need to make stuff up on your own. Oh, reloadersnest, too, tho' I haven't been over there for a long time.

JR1 is giving good advice.

Lots of time and money is spent developing published reloading data by the various bullet and powder companies. There may be a bit of liability conservatism built in, but you can expect that.

You can also find lots of opinions expressed on the internet that have no way of being backed up by developed data. Play it safe and follow published data for your safe reloading.

MCG,

I load blackpowder & bp sub loads in my Open Top replicas (.38 spl) and have noticed the following;

I will load same volume (NOT weight) of bp, and adjust only the inert filler (grits or corncob media), to take up any airspace.

With 125 grain & 158 grain cast lead RNFPs, the 125s will shoot to a LOWER poi than the 158s. I know there is a physics reason behind this, I just can't explain it!

On my full house loads, no filler - just BP compressed slightly, I will get the same effects, although the 125s will have a bit higher poi with more velocity.