Cannot put blade into 310 reciprocating saw the instructions in the manual do not make it clear

The mounting heel of the blade has to be the same style and you have o ensure the blade is firmly inserted deep within the blade holder.

That sounds dangerous! Does it actually cut after you do that? It sounds like it has bad gears and wouldn't cut if you put pressure to the blade. On a dw303 it's likely not worth repair if that's the case.

Here is a parts diagram. Item 52 is the ball. It fits behind item 49. Item 49 has a groove machined into it so that when it is moved it allows the ball to move away from the blade far enough to insert the blade. Once the lever is released the ball moves back and locks the blade into place.

DeWalt parts diagram

lock the blade in a vise or grip it tightly and with the clamp open TAP on the shoe to pull the saw away from the blade. Use gloves and unplug the tool before attempting.

To install blade into saw:
1. Loosen set screw.
2. Insert blade shank from the front between the reciprocating shaft and the blade clamp.
Locate hole in blade over pin on reciprocating shaft.
3. Tighten set screw. If the blade should break off and the shank does not come out of the clamp,
be sure the set screw is loose and eject the shank with a nail or pointed object.

The blade clamp collar is a spring loaded knob of sorts that holds the blade in place. If you are not able to pull the blade shaft out of the saw to get at it, plug the saw in and hit the on/off trigger a few times to get the shaft and knob out of the body of the saw. Once you can get at the knob it should be easy to turn it and insert a new blade. Let me know if you have problems.

put a little bit off LOC-TITE on screw, worked for me

Here you go click on the links below for the pdf file. I hope this helps, please rate if the answer provided has helped. Good Luck and have a Great Weekend.

Parts Breakdown
Product Manual

There is a ball lock in it and you have to twist the ball lock but if the ball lock is frozen generally WD40 will help unfreeze it and work it until the ball is free and should work just fine.

If you see a set screw where the blade goes, loosen it a few turns. Then run the saw, it should spit out the broken blade. If it has a manual release for the blade, press the release while tapping the blade holder.