IIRC, only Bushmaster and Colt make barrels using 4150 steel.  Everyone else uses 4140 steel.  (This correct?)

So what’s the big difference between 4140 and 4150?  Does it make a difference when shooting full-auto vs. semi-auto?  Does 4150 last longer than 4140?

Handles extreme temps better.

10 units of measure



[;D] That was the GD answer.

10

[:D]

actually, temperature tolerance is basically the same for both steels.

4140 has slightly lower carbon content than 4150, making it slightly less hard/lower tensile strength.  Honestly the composition difference between them is less of a factor in this case then how they were heat treated.  4150 is a bit more difficult to machine than 4140, which is why it used less frequently.

On a practical level, both are more than adequite for the job of being a rifle barrel.  With a chromed bore, wear/throat erosion will be the same for either.  Without, i would give the edge to the 4150 but you would be hard pressed to tell the difference in the real world.

4150 is mil-spec... if that does any thing for you...

-Spaceman

4140 has a .40% carbon content while 4150 had a .50% carbon. The 4 stands for chromium and the 1 stands for 1% of that alloy. The 4150 could be hardened more since it has more carbon which would also mean it would have more tensile strength (don't know how this affects its ability to handle heat better but I'm sure it does somehow).

4140 -
Chemistry (%):
Carbon       0.38 - 0.43
Chromium     0.8 - 1.1
Iron         Balance
Manganese    0.75 - 1
Molybdenum   0.15 - 0.25
Phosphorus   0.035 max
Silicon      0.15 - 0.35
Sulphur      o.04 max
Physical Properties:
Density (lb / cu. in.)   0.28
Specific Gravity   7.83
Specific Heat (Btu/lb/Deg F - [32-212 Deg F])  0.114
Melting Point (Deg F)   2580
Thermal Conductivity   23
Mean Coeff Thermal Expansion   7
Modulus of Elasticity Tension  33

4150 -
Chemsitry (%):
Carbon     0.48 - 0.53
Chromium   0.8 - 1.1
Iron       Balance
Manganese  0.75 - 1
Molybdenum 0.15 - 0.25
Phosphorus 0.035 max
Silicon    0.15 - 0.35
Sulphur    0.04 max
Phystical Properties:
Density (lb / cu. in.)  0.284
Specific Gravity        7.8
Specific Heat (Btu/lb/Deg F - [32-212 Deg F])  0.113
Melting Point (Deg F)  2600
Thermal Conductivity   23
Mean Coeff Thermal Expansion  7
Modulus of Elasticity Tension  30

Good info Hopophile. So if they are so close to each other then where did this theory of 4150 handling heat better come from? I understand those numbers don't give any properties of the metal after being heat treated but I don't see how that would affect it much anyways (the 4150 would be a bit stronger but what does that have to do with handling heat)?

There are some differences in specific heat and melting point, even if they are small.  

Bascially the difference is that the military requires 4150, and some places save a little money by using 4140.