I actually have an identical rifle, (Savage M110 in .300RUM).
I doubt seriously that the rifle was harmed. The base of the .300RUM is the same as the belt on the .300winmag which is why you were able to chamber and fire the cartridge. As far as the shoulder moving foward, the .17HMR is designed to do this, so, no problem as all it did was reduce the chamber pressure. I too am suprised by the level of accuracy you got, but, then again, not really.
I knew going in what I was getting into but still had some suprises with the firearm. It was used, and had "issues". The rifle was obviously not properly inspected before it left the factory, as the extractor was installed backwards. Secondly, someone had badly scratched the chamber walls with something hard in ejecting the cases by running something besides a cleaning rod into the chamber. I was able to "fix" both and now have an amazing rifle.
I reload for mine, and although the recoil is "thrilling", it's not as bad as some might think.(think a Rem. Mod 870 pump with 3.5" 2oz, turkey loads...) I installed a "kick-eze" recoil pad and it tamed it considerably. Also, if you get one of the surplus .50BMG powders that are suited to the .300RUM, it can be LESS EXPENSIVE to load for (hence shoot) than a .308wcf or even a .30/30. I bought 16lbs of WC-860 powder for $50.00 from Wideners Shooters supply in '05 and Still have over 12lbs even though I also use it in a .257wbymag I picked up in a similar deal. (I have less than $600 in BOTH rifles....). Currently there is some WC-862(?) on the market for $50 for 8lbs which is still a "deal". This, even at ~100grs per load will make your .300RUM cheaper to shoot than a .30/06 with current "retail" powders.....
If you want to "down" load your .300RUM to the "managed" recoil loads, you have two options. I've used starting loads for IMR7828 for .300WinMag duplication, and got fabulous accuracy (sub-moa). Likewise, you can use XMP-5744 powder to load .30/06 equivalent loads. Hodgdon's even has some loads with "Trail Boss" which take it down to a .30/30 equivalent. However, I haven't tried the latter two options. I've got too many other rifles (.30/30's, and .30/06's) to bother with changing recipes for the .300RUM. Don't be suprised that the "managed" recoil loads don't shoot to the same sight-settings of the full powderd ammo. Mine doesn't. Hence, I don't bother with them. The exception is the mentioned load with IMR-7828. It shoots very close to the "full" powered loads.
For bullets, the Hornady 180gr Interlok is decent as it the Remington 180gr CorLokts. I've used both on deer and they perform well. The Remingtons used to be a lot cheaper than they are now, however. For a good "inexpensive" bullet for target practice and shooting deer, the Privi-Partizan bullets from Graf's are about as inexpensive bullet as you'll find. My rifle likes the 165gr BTSpt and shoots them to about 1.5" at 100yds at about 3,500fps. I'm sure that it'll "splatter" a deer pretty badly, hence I'd expect a bit of meat loss at this loading level. But, it'd make an awesome long-range coyote buster... Think a prararie dog hit with a .22-250 is a spectacular sight? !!!!!!
Enjoy your .300RUM. I do mine, when I'm in a the mood for it......
