any thoughts on the 4.6? im curious as to how long it could stretch to, eg. an inch or more?

within 300m, does BC have much affect?

for example if you had a 140grain 6.5,  versus a  low-BC 22-250. 

would the 22 shed velocity faster

When both bullets have the same BC, they have the same velocity loss. The problem will be to create a .22-250 bullet that has the same BC as a very good 140 gr 6.5 mm. The .22 bullet would need a weight of at least 100 gr to achieve the same sectional density (the highest commercial I know of is 90 gr), which is the most important condition. As EmericD already pointed out, there is not much design space for improving aerodynamics (form factor). By the way,  given an identical shape, the smaller caliber bullet is usually at a disadvantage regarding form factor.

The BC is only really valid for comparing bullets of about the same bullet velocity. In the real world, smaller calibers usually have a muzzle velocity advantage and a BC disadvantage. If you make the bullet heavy for the caliber (for example 100 gr) this advantage gets smaller and could vanish.

In other words, such a question can only be answered when many more additional details are taken into consideration.       

There are a lot of online ballistics computers available, for example on jbmballistics.com to mention but one. When you familiarize yourself with one of these, you can use it to compute the resulting trajectories, kinetic energy at the target, wind sensitivity etc. for yourself. This will not only answer your questions, but at the same time give you insight into small arms ballistics and how things like muzzle velocity, BC, form factor, sectional density etc. interact.  

The best is to play with a few of these online calculators to find one that you like most. Important is that it makes use of the the G7 BC. The older G1 BC (also if only "BC" is mentioned) does not correctly reflect the air drag of modern slender projectiles. In particular, it results in too optimistic velocity figures at longer ranges.   

how often do infantry  have to carry GMPG ammo? does it weigh them down very much.

if the GMPG was mag-fed, with a polymer 6.5mm, would this be easier for people to carry and generally more practical?

assuming a side-mounted 33-38round magazine

GPMG will be belt-fed. How much ammo the rest of the squad carries will depend on the situation. Low chance of engagement, none. Decent chance of engagement, especially with a peer/near-peer enemy, it would depend. Some would carryaspare belt, but others would carry extra ammunition for RPG, Carl Gustav or grenade launchers. Some might be loaded down with LAAWs. Squad leaders, DMs, and "systems operators" would probably have extra comms.

smg762 said:

if the GMPG was mag-fed, with a polymer 6.5mm, would this be easier for people to carry and generally more practical? assuming a side-mounted 33-38round magazine

No.  A side-mounted magazine is less practical than belt feed.  Such a feed system saw limited use by US forces in WW2, but was soon discontinued.

stancrist said:

A side-mounted magazine is less practical than belt feed.  Such a feed system saw limited use by US forces in WW2, but was soon discontinued.

Stan, do you know of any comparative testing of otherwise similar automatic rifles with side, bottom or top-mounted mags? On the face of it, each location has its plus and minus points. And then there was the butterfly twin-mag arrangement used in the WW1 Winchester...?

autogun said:

do you know of any comparative testing of otherwise similar automatic rifles with side, bottom or top-mounted mags?

No, Tony.  I do not.

Interesting point by Karl here

YouTube

SVD vs WWSD - Time & Grains on Target

InRange is entirely viewer supported, please consider it:https://www.patreon.com/inrangetvThe SVD is a notorious Soviet era DMR, which has proven itself on t...

graylion said:

Interesting point by Karl here

Perhaps, if by "interesting" you mean very dishonest.

- Different types of ammo (5.56 match vs 7.62 ball).

- Different types of guns.

- Different types of optics.

"In fact, if you're watching the footage, it's hit, hit.  I'm already sending the second round before I even hear the shooter -- the caller -- call a hit."

ROFL.  That is total BS.  Almost every time, 1-2 seconds elapsed after a hit was called before Karl fired the second shot.