S7 tool steel bolt

[WS6]

What are the possible unintentional consequences of using S7 for an M4 bolt? Is it viable? Now that multiple companies offer them, I'm curious.

[jbsmwd]

Who else is offering them besides Sharps?
http://srcarms.com/relia_bolt.html

[crowkiller]

S7 is used to make jackhammer bits so it should be tough stuff.

[cjb]

S7 is supposed to be a high shock steel, but....

It is grain specific, that is... it tends to be tougher in one orientation, and its coefficient of expansion also runs along the grain. I don't think anything in an M4 would be subject to CE issues... can't say about toughness.

And..... reportedly..... its prone to cracking, and must be carefully minded in heat treatment. I can see bolt lugs being a possible situation where fillet radii might be in order. Again....the word is might, since I have no direct knowledge of how it works in that situation. S7 goes well for things that are large, thick and must take a huge whoppin, like claw tips on excavation machines for instance.

Some quick reference shows that 8620 steel seems to have slightly better surface hardness, will deep harden to about .050 if need be, and still retains good core ductility for toughness. S7 steel, with similar surface hardenss will harden through the part, and have a much harder, and more brittle core.

The question is - what is the surface hardness of the M4 bolt, and how deep are the bolts surface hardness. Do they try to retain a milder core on purpose? The 8620 was chosen for a reason.

[Clint]

Hardness is a double edged sword.

As hardness goes up, strength and wear resistance go up while fracture toughness goes down.

If a part is made too hard and impact loaded, it may shatter in a sudden catastrophic failure.

If a part is too soft, it may wear rapidly and become out of tolerance.


Mil spec bolts are C158, not 8620.

With most any case hardened application, the part attemps to provide a tough inner core of adequate strength with a wear resistant outer case.

Bolts can wear down and fail headspace under certain schedules.

Bolts can also fail from fatigue under different schedules.

[cjb]

I stand corrected, and yes of course C158... temporary oldtimer's syndrome.

However, I see that there is a lot of dis-information on 8620 being a through hardened steel. I don't do heat treating, and used to send out to specailty shops in the day, but... I know that you can 8620 is a surface hardening steel, with deep case hardness if need be. I guess it depends on the thickness, but holding case depth in treatment should be do-able for bolt lug size parts.

The C158... I dunno... I'd have to crack the books, but it very well could be that more expensive on the way in with raw steel, but more favorable in production due to the ease and non-critical nature in hardening treatment. That is to say, if you want to hold case depth to a certain spec, then it may very well be that C158 is easier to do that with, and thus more suitable for military production since there will be lower likelyhood of rejects and bolt failures in the field (ie, M1903 syndrome).

I find this all pretty interesting, as it give me call to go back to the thick books, and try to reverse think why one steel was chosen over another, based not only on what the steel can do, but also what it must endure to become a part thats acceptable in production environment.

[evi1joe]

To answer the OP question, Spikes is about to come out with a dimpled, S7 bolt that has 4 gas-rings--he's been running the SAME style in Carpenter 158 for like 80K-100K rounds with no problems. These new bolts will also be NiB and priced around $124.95...a bit cheaper than the SRC "relia-bolt".
I just got a reliabolt to check out--we'll see how it holds up in a carbine course in March. While I like the IDEA of S7 (if it is so much stronger and less likely to shear lugs or break in half at the cam pin hole), we'll have to see if it holds up to the promises I've heard.

[sinlessorrow]

To answer the OP question, Spikes is about to come out with a dimpled, S7 bolt that has 4 gas-rings--he's been running the SAME style in Carpenter 158 for like 80K-100K rounds with no problems. These new bolts will also be NiB and priced around $124.95...a bit cheaper than the SRC "relia-bolt".
I just got a reliabolt to check out--we'll see how it holds up in a carbine course in March. While I like the IDEA of S7 (if it is so much stronger and less likely to shear lugs or break in half at the cam pin hole), we'll have to see if it holds up to the promises I've heard.

80-100k on a bolt.........bahahahaha what a crock of shit.

[evi1joe]

80-100k on a bolt.........bahahahaha what a crock of shit.

Doh. Well, according to Tom from Spikes, it was 50K-70K and still going (and these aren't S7--they're C158 but have 4 IonBond gas coated rings and are NiBoron): "We have some of the prototype C158 bolts in full auto suppressed Compressor's with insane round counts on them. I know of one rifle that's over 70k rounds now on the same C158 bolt, this rifle has had 2 or 3 FN CHF barrels swaps yet it's still on the same C158 bolt. There's another demo rifle over 50k rounds. We've been testing these features in the Compressor all along and they are holding up without issue. The Compressor would kill gas rings like crazy, up until I started IonBond coating them."

[sinlessorrow]

Doh. Well, according to Tom from Spikes, it was 50K-70K and still going (and these aren't S7--they're C158 but have 4 IonBond gas coated rings and are NiBoron): "We have some of the prototype C158 bolts in full auto suppressed Compressor's with insane round counts on them. I know of one rifle that's over 70k rounds now on the same C158 bolt, this rifle has had 2 or 3 FN CHF barrels swaps yet it's still on the same C158 bolt. There's another demo rifle over 50k rounds. We've been testing these features in the Compressor all along and they are holding up without issue. The Compressor would kill gas rings like crazy, up until I started IonBond coating them."

He can say what he wants but hes just blowing smoke to his cronies who eat it up.

I don't think there is any rifle out there there can get anywhere close to that round count, much less his 6" compressor.