I changed my mind on this half a dozen times. First I thought 12 gauge shotgun with 1/2 slugs and 1/2 birdshot. Changed my mind on that. Wouldnt know what type of round I might need in the chamber at any given time. 22 rimfire to small to deal with large predators. No optics, Id have them broken or out of zero in the first 24 hours. Scratched M-1 and M1-A. Liked the idea of a 30 caliber semi-auto rifle but then I remebered Murphys law, Even considered a drilling with 2 shotgun barrels and a centerfire rifle barrel. Decided since I was limited to 100 rounds of ammo Id rather have all rifle ammo.

 

My final choice is a 1917 enfield in 30/06. Why? Fewer parts to break. Rugged sights. A caliber that is good for deer and will handle most predators. As long as I did my part a hundred rounds would keep me fed and protected for quite awhile.

m1 carbine is damn fine weapon

 

second choice the bushmaster ar-15

It would be a hard choice, but my choice would be either a .22 LR or .30.  You can carry 100 rounds of .22 LR or .30 very easily.  Either rifle would be a bolt action with good iron sights.

 

The .22 LR can be used on pretty much anything.  Because of its small caliber, shot placement is absolutely critical.  That's why good iron sights are a must have on this rifle.  The .22 doesn't make a lot of noise.  Noise can be a problem in some cases -- if you have to escape and evade. 

 

The .30 rifle is OK also.  Ammo isn't as light as the .22 LR and it does make a lot of noise.  Shot placement is critical, but not in the same sense as the .22 LR. 

 

The old M1917 Enfield is tough as nails and homely as a post.  Not a bad choice.  I'd make sure I'd have one spare part for the Enfield -- an extractor, because Enfields break them (mine did) -- and you can carry it in the butt stock cleaning compartment.

 

I've got an old Ishapore-built (1965) No. 2A1 (a 7.62 NATO version of the British No. 1 Mk III SMLE).  It has a Parker-Hale No. 5A match (aperture) sight fitted.  This would be another tough as nails and homely as a post choice for your island escapade.

I must bow to those of you with way more knowledge on this subject than I possess!