Velocity difference: FMJ vs HP

[chrisf8657]

Can anyone guess what the velocity difference in a reloading manual that calls for a FMJ vs a HP of the same weight would be (same barrel length, etc. - nothing different but the bullet type).

PS - NO REPLIES that go into the speil about changing bullet type, etc. etc. etc. I am trying 1 round only.

Thanks

 

[ArchAngelCD]

What caliber are you asking about? Is this in a rifle or handgun?

A FMJ bullet and a JHP bullet of the same weight using the same powder charge should be just about the same velocity from the same gun. One possibility could be if the bullet diameter is different. (.355" vs .356" in a 9mm or .357" vs .358" in a .38 Special) The only other reason I can think of off hand is if the 2 bullets are of a different design where one seats deeper in the case reducing the case volume which will bring the pressure up causing a higher velocity.

How much of a variation in velocity are we talking about?

 

[MachIVshooter]

About the same as standard deviation. Bullet aerodynamics have no effect on muzzle velocity; Whether a super-pointy spitzer or a wadcutter, the bullet still has to displace the same amount of air getting to the end of the barrel.

 

[able]

Ive been trying HPS in 9mm and use +p load data for fmj bullets, I have to ask this question? do hp"s open as velocity goes to 1250 fps and catch air and expand,ive been loading the 105gr. LAWDOG hp but this hp is a fmj profile,polymer inside with lead under and a hollow base,if I load these to +p they dont act as a hp no more their made for low fps and great expansion and poor penetration, I think IF who ever made these bullets would have made the base solid copper then lead covered in poly and kept the serrated FMJ profile these may have been a very good HP, feeds well and dumps all energy when it hits badguy without killing someone behind BG,so profile of bullet and FPS do figure into loading HPs, I ruined a suppressor with high fps HPs,they do expand in a can which I didnt want to happen, my mistake,a 147 gr. FMJ would have been a better choice,DUH on me! hp and fmj cant use same fps, only my experience, a bullet that goes from .355 diameter to .65 diameter has to catch air and really put the brakes on fps. am i just stupid,or is anything i said have any value or have i lost my mind trying to figure the perfect one shot silent 9mm load?I could go into .45 acp? LOL:banghead:

 

[kingmt]

I'm a little confused now. If you have a FMJ & JHP of the same weight, jacket material, & lead alloy then the HP will be longer. If they have the same COL then the pressure will be higher from the same charge.

 

[LightningMan]

You can't tell what all the variables will be when changing bullets of the same weight but of different design. Ok you have a Speer FMJ 9mm of 124 grs. and a Hornady 124gr JHP, with the only data you have being of the JHP. That data gives you an OAL of say, 1.125 (example) but you don't have data for the FMJ using that powder. Now you can measure the JHP bullet's length and subtracting that length from the OAL will tell you how much of that bullet is inside the case or how much volume it takes. Then measure your FMJ bullet (it will probably be longer) so your over all length will have to be longer in order to leave the same amount of volume in the cases capasity. You might thing you would be safe, but you also have to figure how much barring surface each bullets design has, I would think the more surface area engaging the rifling, the more resistance = more pressure. I'm no expert, but when subsituting bullets of equal weight, I would start near the bottom of start charges and work up from there, watching for pressure signs. Good Luck, LM.

 

[gamestalker]

Any variance of a noticable degree, is only going to be realized as external ballistics. MV, even if taken at 20' beyond the muzzle, isn't going to produce a measurable difference. What your really asking, if I'm correct, is BC. External ballistics would very likely be measurable in this respect at longer range, 100 yds. and beyond, depending on the cartridge. But at the muzzle about the only thing that will effect velocity would be a bullet of different diameter, providing they are seated to the same depth.

 

[1SOW]

At the same "seating depth" they will be fairly close.

The only difference that 'can' be significant is the length of the straight sidewall.
Hollow points add the weight removed from the nose back to the body of the bullet. This will affect pressure and bullet speed in the barrel due to increased rifling contact..

A "hollow Base" bullet does the same. A 124gr HBRN is waay longer than a 124gr RN of the same nose-shape. All the removed lead from the base is added back to the body to stay 124gr.

 

[Edarnold]

The answer is there is no answer. The variables in a pistol cartridge include case capacity variation due to bullet intrusion, bullet bearing length, and differences in bore friction due to changes in jacket composition (FMJ would be expected to have a harder, lower friction jacket than a HP)

Witness as an example the 9x19 NATO load, which achieves ballistics at normal pressures that would require +p pressures with most commercial HP bullets.

In addition, the leade in different makes of semi-auto pistols can vary wildly, so that the correct OAL loading for one gun won't chamber in another.

So, the OP has posed a question that is impossible to answer.

 

[1SOW]

Edarnold, This is a reload not commercial ammo. The op said NO changes including powder changes other than FMJ to JHP.
It's relatively reasonable to assume it would be loaded fit his chamber.
The length of the bearing surface will change.
see #8 above

 

[chrisf8657]

Thanks guys - It's for 45 ACP +P.
Two of you could be right - if the bullet is longer and is seated at the same COAL, pressure will rise. I'll adjust accordingly as long as it's within the max allowed COAL. I'm going to change the Ramshot data for Silhouette from a Sierra FMJ to a Speer or Hornady HP.

 

[Usertag]

Well depending on if it is jacketed or hollow based and the caliber. I can't answer this question, but since you didn't specify what type of HP. I will assume it is hollow base and say that the stereo typical jacketed, FMJ rifle round is a bit faster.