For 10 years I worked weekends and summers for a friend who owned three Gun and Police Gear shops. I even got rated as an assistant instructor and Glock Armorer.
We had an event locally where a late teen type had a gun go off in his backpack in class he had his hand in the back pack and dropped it on the floor as it went off - his court defense was it went off when the pack hit the floor and the other students in the room were divided (as normal) on if it fired before or after it hit the floor.
A couple of the local cops and someone from the DA's office came to the shop to get info and advice before the trial. At the time my friend had one of the same model revolver in stock that was so beat up we were going to strip it for parts to sell to a gunsmith and melt the rest.
So he put blanks in the gun and then we proceeded to drop, throw, kick etc. the gun around, both with the hammer cocked and un-cocked. We kept that up for almost 5 minutes including a circle of 4 guys kicking the cocked gun back and forth across a concrete floor for almost 2 minutes - sort of "revolver soccer". We never got it to go off "by itself".

A week later one of the cops came back in and told us that after he and his partner + the guy from the DA testified to what we did without getting it to go off the kid broke down and confessed: 1. he reached in the back pack and cocked the gun 2. he gripped it with his finger on the trigger intending to pull it out to scare someone "as a joke" 3. as he pulled on it the gun caught on something inside the backpack so he jerked harder - that was when the gun went off because he had his finger on the trigger. And the idiot also said he forgot he had loaded the gun then forgot to unload it when he put it in the backpack for his planned "joke". The judge provided a punch line for his joke - one he was not very happy with.

I know of one type of gun (a semi-auto military rifle) that has a part that when worn badly and slammed hard on something butt or muzzle first will go off. It was the early production runs of that rifle, they corrected the problem. A few models of semi-auto pistols with extremely worn down sears can fire when something hits the cocked hammer very hard. But it is rare and then only in guns that are not inspected or cared for and the sears replaced when that badly worn.
At the shop we once got a high end 22 target pistol from a police confiscation auction* that had deliberately and very carefully had the sear filed down and a lighter than spec recoil spring put in so that when fired it went full auto. Everyone in the shop and about 1/2 dozen local cops came in to play with it in our bullet trap before our gunsmith replaced the two parts. I tried it, with 22 shorts it just sort of vibrated as it fired (it was a "heavy" barrel target model) and kept all the rounds on target at 5 yards. And it emptied the 10 round magazine in just over one second. After fixing it so it was legal again a part timer at the shop bought it and then proceeded to win some competitions at the local gun club.

*The department it was purchased from said the guy they took it from was supposedly a hitman for a local drug gang. But they only got him on felon in possession and possession of an automatic weapon. No known hits/bodies shot with it had ever been found. But the two other charges got him 10-20 years and the state judge ruled they would be served after the Feds were done with him. The Feds tried and convicted him on the same two charges and they also gave him 10-20 Federal time. I doubt he got any time off for good behavior at either prison so he would have done a total of 40 years.

But that makes it full-auto only, besides the selector is a pop on - pop off part that requires minimal tools and skills.

The M-14 is still used for some purposes, a marksman version, an enhanced long range version with bipod etc. that in some units, esp. in Afghanistan with long range fire fights being common, was issued one per squad. (Because the 5.56 NATO is not a great long range round.)

My issue weapon the whole time I was in the Army was the M-14. When my company (a HQ unit) was training to ship out to Nam the CO made the decision that any officer* or any NCO E-6 or above could have a selector if wanted. For the rest of the company only the top 10 qualifying shots would get selectors. I was #2 behind an 18 yo kid from the Ozarks who grew up very poor and from about 8 was hunting game for the cooking pot. And I wore/wear glasses like Coke bottles. One of our NCOs who had been on the Army Marksmanship Team taught those of us with the selectors how to control the M-14 on full-auto. From the prone or foxhole position wrap the sling around your supporting arm and then lean forward using your whole body weight to hold the weapon down and fire in 3 round bursts. Some of us managed to get hold of some clip on bipods meant for the M-16 and those made it even easier to control when prone, foxhole or firing over a wall etc. where you could rest the bipod. But since we were the 10 best shots in the company we were told "We want you acting as sharpshooters on semi-auto. Save the full-auto for Charlie is in the wire at 10 meters."

*HQ unit for a support Group, we were about 25% officers of just under 150 personnel. During field training we were organized with one officer per platoon and all the extras became a light platoon of nothing but officers, commanded by our Group XO, a LtCol with our Group E-9/Sgt Major acting as his platoon Sgt. And yes, that meant during field exercises he was bossing around officers up to Major in that platoon. Our Group CO, a full Col, acted as the company CO with the Captain who was the company CO acting as his XO. And we had so many senior NCOs that fire team leaders were E-5 & E-6 while squad leaders were E-7s and the platoon Sgts E-8s.

The Seabees in Viet Nam were issued M-14's until ca. 1970. When I got off active duty in the Navy, I stayed in the Navy Reserve for several years, and during that time, I used an M-14, sans selector switch, for National Match competition at Camp Perry. Our rifles were supplied by the USMC Reserve, who occupied the same building we did. They had converted to M-16's, but had 20, or so, still in the armory. I like the M-14 better, especially at the longer match distances. Most had barrels with excessive muzzle wear, and/or excessive head space problems. However, we could get them rebuilt by the Navy Gunner's Mates, or the Marine armorers at the competition. The first year mine got a new barrel and trigger group, then later on a rebuilt gas system. We used those until 1974, when the Marines had to return all M-14's to the central armories. I was issued a "new" M-16 A2, but it just wasn't the rifle the M-14 was.

I qualified as "expert" with the M-1 in basic training and the M-14 (in semi auto mode) in AIT. Also had to qualify separately with the M-14 in full auto. I barely qualified as marksman with that beast. Totally uncontrollable. The bipod made a good prone rest for the fist shot, after that the damned thing was all over the place. I knew guys that were fairly proficient with the E-2 version but I never knew anyone who could shoot the standard "14" decently.

Never even saw an M-2 though I owned briefly one of those Universal Arms "sporterized" M-1s. I enjoyed shooting it, but it wasn't a particular caliber that I felt I had a use for so it went away the way it came, in some trade of other.