Bullet length

[leadchucker]

I can almost always find load data for a particular size and weight of bullet in one particular type, such as a JHP, but I can't always find load data in that same weight for any other type, such as an FMJ. Powder charges for these two bullets would likely be close enough to interchange between bullet types, but because of the difference in nose shape, the OAL is likely to be significantly different. I'm speaking of average pistol loads.

I know that if you know two of the three parameters for a particular bullet, OAL, seating depth, and bullet length, you can always calculate the third. So if I just calculate the bullet seating depth of the JHP, based on OAL and bullet length, I can apply that seating depth and the bullet length of the FMJ bullet, and come up with an acceptable OAL for the FMJ load.

The OAL for the particular load and bullet is always in the load data. But the bullet length never is. If you don't have an example of the specific bullet mentioned in the load data, it's impossible to know the length of that bullet, unless you can contact the bullet maker and ask.

Is there any other fairly reliable method of figuring OAL on a bullet like this?

 

[rcmodel]

Yes there is!
You HAVE to find the correct length by doing the "Plunk Test" in your barrels chamber.

Bullet length inside the case isn't the problem.

The part that sticks out too far and jams in the rifling leade if seated too long is the problem.

Once you find that OAL for your barrels chamber?
Use the Start Load and work up, watching ejection patterns, proper functioning, etc, to determine when you reach your guns MAX load at that OAL.

rc

 

[LiveLife]

rc beat me to it.

 

[leadchucker]

I don't think I have ever created a round that failed a plunk test. That's not the issue at all for me.
What I'm more concerned about here is pressure, which is directly related to internal chamber volume. which is directly related to bullet seating depth.

Pistol ammo. 45 acp, 40 S&W, 9mm Luger, 380 acp.

 

[rcmodel]

Well, I see you have it all under control then.

Sorry I bothered you.

rc

 

[Skinnedknuckles]

This may not be the best way, but it is what I've followed to date. First, I ASSUME that an FMJ load is available and that all RN FMJ bullets of the same weight are essentially the same length. I then compare the length of an FMJ of that weight that I have "in stock" to the bullet I want to work up a load for. I then use a COAL that much different (usually shorter since FMJ's seem to be the longest bullet) from the published length for the FMJ, and use the powder weight recommendations for the FMJ. This should keep the useable case volume the same and since the bullet is shorter I don't expect transient pressure spikes from the bullet "sticking" in the lands. Obviously it doesn't work for hollow base shapes. If the only published data is not an FMJ but instead is a brand name, I would ask around the gun club see if someone had that bullet, but I haven't had to yet.

So far I haven't had any feeding problems and velocities measure in line with the publish load data, so I feel safe. If I'm making a mistake, I'd appreciate being corrected. I load 9mm (and .38 Special but length isn't as important there).

 

[gamestalker]

Pressures are relative to seating depth, which is determined by the "plunk test" while working up from minimum data for any jacketed bullet of equal weight reagrdless of what style jacketed bullet is concerned. In other words, JHP and FMJ would utilize the same minimum starting charge for the same weight bullet.

So olgive location is what determines the OAL by use of the "plunk" test. Proper seating depth FOR YOUR FIREARM will always be determined by the lands to olgive distance, and has nothing to do with how long the bullet is from nose to base.

Pressures are adjusted accordingly by working up from minimum, and then only after having identified the proper seating depth for your firearm.

I understand what it is you are asking, but it has nothing to do with determining the seating depth or OAL, that is regulated by the distance to the lands from the olgive of the bullet, not how deep the bullet seats into the case. Although depth into the case has an effect on pressures, pressures are not managed from that perspective. Pressures are managed by the powder charge work up after having determined the ideal seating depth, or OAL by means of the "plunk test", distance off the lands, and magazin fit.

GS

 

[918v]

I think the OP is trying to use JHP case volume as a guide to determine the OAL for a FMJ. This is a bad way to go about it. It will result in the FMJ seated way too deep in the case and cause feeding problems.

FMJ are generally seated long. JHP are generally seated short. Reloading manuals often have the same charge weight for both bullet types, but that data defers to the JHP. In other words, the max charge weight is not really max when used with the FMJ because the FMJ has more case volume available.

 

[jim243]

You can measure the two bullets and adjust your OAL based on that difference, that would give your the internal volume needed, HOWEVER because one bullet is longer than the other, the bearing surfaces on the two bullets are different and will provide more resistance on the FMJ going down the barrel than the other bullet and produce a higher pressure for the longer one than the other eventhough the internal case volume is the same for both.

To be on the SAFE side, I would start at 10% less powder than MAX and work up the load checking for pressure signs as I go. This may be your only alternitive if no data is available for the bullet you are using.

Stay safe and go slow.
Jim

 

[918v]

If you seat the FMJ to the same seating depth as a JHP, you will likely place the case mouth above the ogjive. You'll likely make a round that looks like the one on the far right:

 

[leadchucker]

918V, that round on the right is exactly what I'm wanting to avoid. In addition to it likely causing feeding, and crimping problems, the round's internal volume is severely reduced. Enough to be concerned about increased pressure.

Have no doubt, I'm working up from a comfortably safe minimum charge weight, regardless of what OAL I opt for. What I'm afraid of is that even with a ten percent or more reduced starting charge, pressure could be dangerously high if the internal volume is too small.

 

[leadchucker]

I think the OP is trying to use JHP case volume as a guide to determine the OAL for a FMJ. This is a bad way to go about it. It will result in the FMJ seated way too deep in the case and cause feeding problems.

My goal here is to achieve the same internal volume with either bullet, by matching the seating depth. The FMJ bullet is longer than the JHP bullet, so if the seating depth is kept the same, the round OAL will be longer too. Naturally, I don't want to seat the ogjive lower than the mouth of the case.

That may not be the perfect solution, but I think it would assure that I don't exceed pressure limits, especially if I work up the load with a comfortably safe starting load.