Melonite vs. Chrome Lined Longevity

[Junkie]

I was wondering why nitrided 4150 is farther down the list than plain 4150. I understand that you can get better quality rifling on a stainless barrel. I thought the free machining 416R resulted in slightly better rifling than standard 416, but I could be wrong on that.

[BillyJack556]

Faxon produces 416R QPQ barrels. My two are very accurate and I have no doubt the QPQ treatment will increase longevity. It also seems to give a slight increase in velocity.

[ColtSeavers]

I was wondering why nitrided 4150 is farther down the list than plain 4150. I understand that you can get better quality rifling on a stainless barrel. I thought the free machining 416R resulted in slightly better rifling than standard 416, but I could be wrong on that.

It's not lower on the list because it is less accurate, but because it is as close to exactly as accurate but with more durability/a longer lifespan.

[VIP3R 237]

Faxon produces 416R QPQ barrels. My two are very accurate and I have no doubt the QPQ treatment will increase longevity. It also seems to give a slight increase in velocity.

Same here, I think Nathan said a life of 8-14k rounds expected life with a qpq 416r.

[FaxonNathan]

Faxon produces 416R QPQ barrels. My two are very accurate and I have no doubt the QPQ treatment will increase longevity. It also seems to give a slight increase in velocity.

That is correct. QPQ (or any of the nitrocarburization processes) imbunes Nitrogen into the steel, which increases surface hardness, increases dry lubricity, and significantly increases corrosion & abrasion resistance.

As such, it results in the mentioned increase in longevity and velocity.

Plus, it looks great!

[FaxonNathan]

ColtSeavers, nice rundown. Now the question to you is how much more longevity in your order of barrel materials and finishes.

In our experience (using semi-auto fire, AR-style weapon):

4150 QPQ or Chrome**
4140 QPQ or Chrome
416-R QPQ
---Note there is a huge gap here in terms of longevity---
Bare 4150
Bare 4140
Bare 416-R

*In a non fully-automatic firearm, the difference in longevity in a bore between QPQ/Nitrocarburization is roughly the same to a "standard" chrome lining. QPQ will have more corrosion resistance through all stages of the barrel, where once chrome is worn through, the base metal is exposed.

Chrome can be longer-lasting if applied "thick" which is very uncommon in MSR barrels. This is common to see in Machine Gun barrels (which we also manufacture). However, a thick chroming is very difficult to maintain MOA accuracy (again, why its used in Machine Guns).

[FaxonNathan]

I haven't seen any claims of nitride reducing the accuracy of a barrel. While it makes it far more wear resistant, it doesn't really change dimensions. I could be wrong though.

That is correct. QPQ as a process processes existing steel versus adding a coating on top of a steel. As such, dimensional change is negligible (assuming stress-relieved steel).

[FaxonNathan]

How does chrome lining bond to nitride steel? Is it compromised at all from the carbon or nitride?

We have not heard of this before. Primarily because chroming uses an "electropolish" in its process that eats away at the bore which the chrome replaces during coating.

Would largely defeat the purpose of a Nitride process.

[lysander]

We have not heard of this before. Primarily because chroming uses an "electropolish" in its process that eats away at the bore which the chrome replaces during coating.

Would largely defeat the purpose of a Nitride process.

You can chrome plate over nitriding. It has some quirks, but it can be done and it will adhere well.

And, since nitriding does not add material you could do the electro-polish on the bore before nitriding.

The M256 120mm tank gun operates at 87,000 psi with propellant temperatures of 3300° to 3500° F, these are nitrided using an ion nitride process, then chrome plated. They still only last 1,500 effective full charge rounds....

[FaxonNathan]

You can chrome plate over nitriding. It has some quirks, but it can be done and it will adhere well.

And, since nitriding does not add material you could do the electro-polish on the bore before nitriding.

The M256 120mm tank gun operates at 87,000 psi with propellant temperatures of 3300° to 3500° F, these are nitrided using an ion nitride process, then chrome plated. They still only last 1,500 effective full charge rounds....

True, but would come at significant cost. Electropolishing is done at the chrome plater so unless your chromer is the same as your nitride house, there are significant lead-time and cost challenges.

I can say we (Faxon) would not likely take this path unless one of our OEM customers asked us to do so for them. Just can't see the major benefit, especially with mis-match in abrasion resistances once the chrome is worked-through.