With the Rolling block chambered in cartridges such as 8mm level and 7x57 Mauser, what is the maximum chamber pressure the action can handle? I have seen sources say the 8mm Lebel has anywhere from 46,000-51,000 psi, and I have seen sources say the 7x57 has 42,000-46,000 psi.  Considering the bolt thrust goes down with smaller case head sizes would it be unreasonable to convert a Remington rolling block to a 5.56 or 6.5 Grendel? By my rough calculations the 7x57 has more bolt thrust at 42,000 psi than a 5.56 at 55,000 psi. If headspace and dimensions are all correct could a rolling block with modern materials handle a smaller, yet higher pressure round?

I've answered this question (and been beat up over it) so many times I'm sick of it, and I almost just passed it over.  But, the answer is, as Zack said, NO.   

The Rolling block action doesn't handle gas leaks very well.  It was designed for pressures in the 30,000 psi range.   

Find a suitable action if you want a modern, high pressure cartridge.  If you just want to build a rolling block, build it in a suitable, low pressure round.

RollinRollinRawhide wrote on Jan 24^th, 2017 at 6:04pm:

There is some debate as to whether the 7mm rolling blocks were actually what we today call the 7x57. There are small differences in the case dimensions, as I recall, and I think Remington supplied the ammo for them, so who knows the true pressure.

As for the Lebel, I recall it was issued to troops well behind the lines who were not expected to fire many rounds ever with it. Are you sure what was the service life? Also, for which generation of the Lebel are your pressure figures? The last ones were extra hot for use in certain machine guns.

My rolling block is of modern steel, in .30-40. I'll never fire factory ammo in it, and my reloads are in the very low 30s only.

In about 1966 I read an article in Gunsport magazine about a 45-70 built on a 7x57 rolling block and just had to have one.  I found an excellent action at the time for $20 at a gun show. I was interested in the 45-70 as a powerful deer rifle so of course wanted to know how far I could go before I killed myself.  I wrote to P.O. Ackley who replied that the practical pressure limit was when a fired case turned 180 degrees would still chamber.  He went on to state that when the action failed it cracked at the pins.  You can find pictures of some spectacular failures like this on the Internet and at least one was fatal.  There is a lot of "give" in a roller and the case will bend some on firing.  I used QuickLoad to see what kind of pressures my loads developed and none exceeded 33k PSI (piezo).  I would not shoot these in my old age, but youth is invincible.  A few years later while in grad school I saw a RB at the LGS chambered in 22-250 for $50.  I saw it as another 45-70 but bought a box of factory 22-250s anyway to see how it would shoot.  Remember, youth is invincible.  The case head bent severely and the case ruptured ahead of the web.  I did get a very nice one shot group.
Today I have several rollers.  I have a 45-70 on a smokeless action and it will never see the 33k loads of the old days.  I shoot smokeless in all of my originals, even my #2 in 38W, but I would never consider a high pressure load as safe in any roller, modern steel or not.

I would add that even a perfect action will have enough stretch in it that your cases will only last a few loadings at smokeless pressures. 

If you want to chamber an old BP rifle for a hot smokeless round, look for a falling block design.  My 2 cents.

jn