Nothing like this has been posted here before that I know of, so folks were talking about it on another forum I read and thought someone here might find it useful.

This chart breaks down how many shells equal a pound for each caliber.

Interesting tidbits: AK-47 ammo weighs about 1 pound for every 27 rounds where as .22 ammo is 133 rounds per lb.
So basically you could carry 100 rounds of AK ammo which is around 3.6lbs or 500 rounds of .22 at 3.7 lbs. lol maybe the .22 survival gun enthusiasts are on to something.

Pistol Calibers

.380
Rounds per pound: 47.06
Weight per 100 rounds (lbs): 2.13

9mm Luger
Hornady 115gr JHP/XTP
Rounds per pound: 38.10
Weight per 100 rounds (lbs): 2.63

.38 Special
Rounds per pound: 34.78
Weight per 100 rounds (lbs): 2.88

.357Mag.
Remington UMC 125gr SJHP
Rounds per pound: 30.77
Weight per 100 rounds (lbs): 3.25

.357Mag.
Handload 158gr JHP
Rounds per pound: 28.07
Weight per 100 rounds (lbs): 3.56

40S&W
Rounds per pound: 28.07
Weight per 100 rounds (lbs): 3.56

.44magnum
200gr Hornady XTP HP
Rounds per pound: 22
Weight per 100 rounds (lbs): 4.57

.44magnum
240gr LSWC Bullet
Rounds per pound: 19.7
Weight per 100 rounds (lbs): 5.07

.45ACP
230gr Winchester Ball
Rounds per pound: 21.33
Weight per 100 rounds (lbs): 4.69

RifleCalibers

.22 LR
Remington Golden 36gr PHP
Rounds per pound: 133.33
Weight per 100 rounds (lbs): 0.75

.223/5.56X45
(milsurp) British Radway Green SS109 63gr
Rounds per pound: 37.21
Weight per 100 rounds (lbs): 2.69

30-30 Winchester (a.k.a. .30WCF)
Winchester Silvertip 170gr flat nose
Rounds per pound: 20.28
Weight per 100 rounds (lbs): 4.92

.243Whinchester
75gr Hornady V-max Handloads
Rounds per pound: 22.22
Weight per 100 rounds (lbs): 4.5

.308 Winchester
Remington UMC 150gr FMJ
Rounds per pound: 19.05
Weight per 100 rounds (lbs): 5.25

.308 Winchester
168gr BTHP Match Bullet
Rounds per pound: 18.67
Weight per 100 rounds (lbs): 5.35

7mm Remington Magnum
Winchester 175gr Power Point
Rounds per pound:14.68
Weight per 100 rounds(lbs):6.81

7.62X39
Wolf Steel Case 122gr FMJ
Rounds per pound:27.59
Weight per 100 rounds(lbs):3.63

Shotgun Calibers

12GA 2 3/4" Slug
Federal HI-Shok Slug
Rounds per pound: 10.53
Weight per 100 rounds (lbs): 9.50

12GA 2 3/4" #4 Shot
Remington Express 4BK
Rounds per pound: 9.30
Weight per 100 rounds (lbs): 10.75

12GA 2 3/4" #7 1/2 Shot
Federal #7 1/2 Shot
Rounds per pound: 10.53
Weight per 100 rounds (lbs): 9.75

12GA 2 3/4"00 Buckshot
Federal Express 9 Pellet
Rounds per pound: 9.76
Weight per 100 rounds (lbs): 10.25

12GA 3" Slug
Federal 3" Rifled Slug
Rounds per pound: 8.89
Weight per 100 rounds (lbs): 11.25