Why not have 77gr 5.56mm as a standard cartridge?

[kal]

I wonder why hasn't the military start phasing out 62gr M855 for something similar in 77gr or heavier? I can't see it being a logistical problem, it's just switching out the bullet.....right?

[Elxx]

You have to change the amount (or even brand) of powder to maintain the accuracy and energy. So more powder plus heavier bullets = more money. Plus, contracts have been signed for x-brand powder "lots" with certain bullet weights from particular companies. Its a lot of work for what the military would consider to be "negligible improvement."

[markm]

There's less of a given powder in a cartridge when you move to a heavier bullet.... not more.

[Elxx]

My experience with reloading is limited to 308. So what you said could be possible.

With the 308, the heavier round usually needs more powder to maintain decent velocities. On the other hand, longer rounds sometimes can't use more powder because of reduced case volume.

If you are right about heavier bullets using less powder in 5.56, I'd be excited to learn why.

[Keesh]

Reduced case volume means increased pressures. Also pushing a heavier round out, you will be increasing pressures.

[kal]

Reduced case volume means increased pressures. Also pushing a heavier round out, you will be increasing pressures.

but a 77gr round works out anyway, and were used in theater.

And for the discussion, I was thinking about increasing the length of the M855 ball to get it to 77gr~ in order to increase vehicle penetration (and maybe other barriers) instead of going towards a different caliber altogether (6.8spc).

I wasn't talking about OTM type munitions.

[chadbag]

If you are right about heavier bullets using less powder in 5.56, I'd be excited to learn why.

He is right. Heavier bullets are usually longer bullets unless we are talking about totally different materials and construction. Since the diameter is fixed, as we add material to make things heavier they also get longer. That equates to less room for powder. Smaller spaces for burning powder to expand into increase rate of pressure increase and can increase overall pressure. They are also slower down the tube which increases pressure as well.

Btw, this is also true of other things like 308 depending on powder you are using and case capacity etc. It is not a 556 specific thing. If you have extra space in a 308 case and go to a heavier bullet you may be able to get away with more powder. But that would probably be the exception and depends on powder and bullet being used.

[Keesh]

but a 77gr round works out anyway, and were used in theater.

And for the discussion, I was thinking about increasing the length of the M855 ball to get it to 77gr~ in order to increase vehicle penetration (and maybe other barriers) instead of going towards a different caliber altogether (6.8spc).

I wasn't talking about OTM type munitions.

Sorry for the confusion, I was posting in response to the poster above my original post.

Just upping the weight of a projectile doesn't necessarily give it better penetration, especially when you are limited to a 223 case. M885 is being replaced now by M885A1. Another limiting factor may be barrel twist rates currently in use.

[kaltesherz]

62gr is the NATO standard, plus all ACOGs in service are calibrated for M855. The USMC was at least smart enough to switch to the Mk318 round in OEF recently, where the Army screwed the pooch and is still pushing the M855A1 round... talk about a step backwards.