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A few questions. Sorry if these are a redo.

Are the barrels threaded on all M&P 22C's? Yes

Do they come with a thread adapter? No. Several manufacturers make them now, including EWK Arms, TROS, Twin Tech Tactical, SRI.  I recommend EWK Arms.

Is the safety removable like the FS? Yes, although I understand the procedure is a bit different.

Will the compact fit in my FS M&P 9 holster?  No

Are the compacts accurate enough for steel challenge? ...vs a Buckmark? Can't help you there; Never shot a steel challenge.  It is, however, a 3.56" barrel, with an attendant short sight radius.

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Silly question but curious, rail fit a X300?

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The 22 Compact is a simple concealed Single Action hammer system.  Taking out the safety is like taking out the safety of a 1911 .45 Auto without even having a grip safety.

The guts are unrelated to the M&P line.  The .22 is not a striker fired double action.

If you grasp the machinery, taking out its only safety seems silly.

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As an interesting monkey wrench in the comparison assuming the purpose was to find a training aid for the owner of a S&W M&P full size auto pistol, in 2013  I bought a Umarex "medium" M&P CO2 BB pistol and 5 magazines.  
The plastic pistol is a dead ringer size wise for a standard M&P.  The trigger function is similiar in that the BB pistol is simply a short DA pull to fire without other controls.  The sights are decent 3 dot copies.  A CO2 cylinder lasts for 4 of the 18 shot BB magazines.  A normal M&P holster is a perfect fit.  Draw and fire practice on an IPSC target is easily done keeping all hits in the A zone through 20 yards.
More fun is to put three pop cans on three vertical pieces of electric wire about a foot above the ground.  Draw and fire three shots. The cans rattle and clang around if hit.   If you hit two of the three, one shot at each, take a step backward.  Repeat.  Hit two of three and take another step back.  At about 20 yards it gets difficult.
When .22s were (are still mostly) unobtainable, a milk carton of BBs lasts a L O N G time.
I liked the set up enough I bought a second one.
Setting up both BB pistols required two things.  First, both shot high so I reduced the height of the rear sight and they shoot to zero at 15 yards.  I simply turned the pistol upside down with the front sight on a slick plastic pad and the rear sight flat on a fine bastard file.  Back and forth a few times reduced the rear sight height.
Second, the top of the trigger where the barrel is activated needs lubed for a smooth pull (and lighter weight).
With those two steps, the sensation is a normal M&P trigger or a Shield trigger that still needs lubed and broken in.  Both pistols show no sign of damage or wearing out.
Still seeing no accuracy info on the .22 Compact besides a 2" at 10 yards or 5" at 25 yards reference, I am not in a rush to buy one.  Considering the safety style SA action of the .22 Compact, the BB pistol is a closer action copy of the M&P system than the .22 is.
I've used the Umarex to introduce new pistol shooters to the idea very adequately.
The BB pistol is a full size copy.  It is too big for my granddaughter to function.  So I bought the SR22P which was a nice first step.  I started her with a CB Long single loaded which goes pop once and nothing more.  She got the hang of it.
I am no fan of the cheap .22s sold by the Walther/Umarex/whoever group, but freely admit the Umarex BB pistols are extremely nice.
I suppose this will get laughed at a lot, but its why they put doors in boxes so you don't have to stay inside all the time.  Trigger time is trigger time.  And with no noise or recoil, you can see your good ones and your bad ones.
Other Useful Info:
Umarex site:  http://www.umarexusa.com/Smith-Wesson-M-P-Black.html#.VExQVVeDe-I
Amazon:  http://www.amazon.com/gp/product/B005116QN8/ref=pd_lpo_sbs_dp_ss_1/179-7283556-6990525?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=lpo-top-stripe-1&pf_rd_r=11DF6R7XJV7T0F7NHFK6&pf_rd_t=201&pf_rd_p=1944687442&pf_rd_i=B00193TWYW
They go $35-40, sometimes with and sometimes without extra shipping.  A lot more $ in stores.
Magazines:  Amazon or Walmart:  http://www.amazon.com/Beretta-Elite-Crosman-19-Shot-Stick/dp/B001BR2YTQ/ref=pd_sim_sg_3?ie=UTF8&refRID=0GRJG8B45HKEW9ERBM5G
Pellet Gun Oil:  Always put a drop on the seal area of the C02 powerlet.  If you don't, the seal may leak and will wear out for sure.  Buy this if you need the extra penny for free shipping:  http://www.amazon.com/Crosman-Pellgunoil%C2%AE-Air-Lubricating-ounces/dp/B0002IL6M6/ref=pd_sim_sg_4?ie=UTF8&refRID=0GRJG8B45HKEW9ERBM5G

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I'm well over 1k thru mine. Only issues I have is when it's dirty and I try to run cheap bill ammo thru it. Usually the first round or two might fte. Cci or fed auto match run it like a champ.


<a href="http://s16.photobucket.com/user/evnash/media/Guns/F0499069-5723-4C0A-8261-CDA7E3B724C3.jpg.html" target="_blank">http://i16.photobucket.com/albums/b28/evnash/Guns/F0499069-5723-4C0A-8261-CDA7E3B724C3.jpg</a>

I've made several holsters for the 22c. IM if you need one

<a href="http://s16.photobucket.com/user/evnash/media/Guns/FullSizeRender1.jpg.html" target="_blank">http://i16.photobucket.com/albums/b28/evnash/Guns/FullSizeRender1.jpg</a>

<a href="http://s16.photobucket.com/user/evnash/media/Guns/FullSizeRender2.jpg.html" target="_blank">http://i16.photobucket.com/albums/b28/evnash/Guns/FullSizeRender2.jpg</a>

<a href="http://s16.photobucket.com/user/evnash/media/Guns/94484D8B-8900-4B2B-8FE8-6657B3E6392C.jpg.html" target="_blank">http://i16.photobucket.com/albums/b28/evnash/Guns/94484D8B-8900-4B2B-8FE8-6657B3E6392C.jpg</a>

<a href="http://s16.photobucket.com/user/evnash/media/Guns/5729389F-B6E8-4C29-AD71-8A95967C8798.jpg.html" target="_blank">http://i16.photobucket.com/albums/b28/evnash/Guns/5729389F-B6E8-4C29-AD71-8A95967C8798.jpg</a>

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Are the barrels threaded on all M&P 22C's? Yes

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What is the accuracy and trigger pull of these?

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BTW if anyone wants to fit 15rnds in the magazine you can replace the follower and spring, but you lose the loading assist button and magazine hold open.

https://www.youtube.com/watch?v=dL18D2znvlM

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From your question, it seemed apparent you have not seen how the .22 operates.

Describing the machinery after seeing the actual parts is not assuming.  It is just a description.

Removing the thumb safety removes the pistols only user operable device to prevent it from firing.  I call that silly to be polite.

The .22's single action rotating hammer is unlike the M&P striker system's long pull trying to copy Glock's.

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Nice pistol, got about 200 rds through it, no problems.

http://i1296.photobucket.com/albums/ag1/bradwd/C22_zps3466df52.jpg