Further info from internet sleuthing.
.300blk case length is less than 1.4", so it would qualify. Also:
Mr. KRINK85 (real name omitted):
Using that cartridge in an AR pistol is legal for taking deer during the firearm seasons as long as the weapon is set up as a single-shot only. This can be accomplished by the use of a magazine with a single-shot follower installed or a sled. This particular follower does not allow any rounds to be seated within the magazine. It simply acts as a "sled" to hold the round until chambering.
Another way to do this is to use a lower specifically machined as a single-shot receiver. This part will have no provision for inserting a magazine. Most hunters will not elect to use this method simply because it's too single-use and permanent.
If you've modified the AR in one of the ways described above (to make it a true single shot), you do not have to remove or modify the gas tube in order for it to be used for deer hunting in IL. The law states that semi autos are illegal to use but that only pertains to loading. Automatic ejection of the spent cartridge makes no difference.
Note that you cannot legally use the AR pistol if all that you do is remove the gas tube. Even though removing the gas tube would render the pistol a single-shot, using the magazine to hold additional rounds is actually illegal. AR and bolt-action pistols must be set up as single-shots incapable of holding additional rounds in any on-board magazine. Internal or external box magazines are not legal for any pistols used in IL to hunt deer.
Thanks for your interest in Illinois deer hunting.
Paul Shelton, Manager
Wildlife Programs Section
IDNR