I have a couple booklets on how to clean an M1A. They were before my time.

The booklets write:

13. A #15 long drill bit with handle. This is needed to clean inside the gas piston tail.
14. A letter “O” drill bit. This will be used to clean the inside of the gas plug.
15. A letter “P” drill bit. This will be used to clean the large opening in the gas piston.

Next, clean the inside of the gas piston with a letter P and number 15 drill bits.

I was at two hardware stores today, one being Menards, which is a midwest competitor to Home Depot and similar store.  All of their bits were either metric 4/32 or the English System, such as 1/4 inch or 5/16 inch.  Where does all this #15, O, and P come from.  I even asked the fine folks that worked there and they didn't know.

O, P, 15 are sizes, the O and P can be jobber length an aircraft #15 will get you deep into the tail of the piston.

Here is the #15.
https://drillsandcutters.com/15-x-6-hss-aircraft-extension-drill-bit-drill-america/
Years ago I used to make cleaning tool sets.
Attached File

#15   long drill bit   #15 drill bit = 0.1800 inches

Letter "O"  drill bit   O   drill .316 in.

Letter "P" drill bit       P drill .323 in.

Any machinist or tool and die guy will know that in addition to fractional size numbered drills there are sets of Letter drills and sets of Number drills.  If you look at tap drill charts you can see that these fall in between the sizes of the standard fractional drills    Tap Drill Chart

For the most part you can clean those areas with a brush with a patch on it.

Creedmore Sports Vented Gas Plug is a good idea
note also that heavy bullets like 180 gr can bend the Op rod  stay under 168 gr.

And to  make it even more confusing there are Reamers and then Metric and high speed tool blanks to grind your own tool .

https://www.brownells.com/gunsmith-tools-supplies/rifle-tools/gas-system-tools/m14-m1a-gas-system-cleaning-drills-prod40378.aspx

Letter and number drills are available from industrial and machinist suppliers. The #15 for the gas piston is an aircraft length drill bit not a jobber or mechanics length.

MSC

McMaster Carr

Grainger

Travers Tool

etc...

What kind of powder gums up/builds up inside the piston/gas tube on an M1A?

My M1A is approaching 43 years old later this year.  I have never had to run a drill bit into anything on it.  

I shoot mostly reloads with IMR powder.  I know Winchester powder is both dirtier and expands the group size, at least in my M1A, so I don't use it.

I clean the gas system after shooting it.  Remove the cap, the piston and use powder solvent (mostly Hoppes#9) to wipe out/wipe off the surfaces.  Then a light coat of oil and it goes back together.  I have shot a few thousand rounds of South African and Czech 7.62X51 and .308 (the Czech stuff is corrosive) but again, I cleaned it after every day's shooting.

Curious how many rounds people go without cleaning one to result in having to run a drill through it to remove the built up/hardened powder residue.

I bought the set I posted above, because it was recommended.

I haven't had to use them yet.

Like you I clean mine with hoppes and put a light coat of ballistol on it.  I do it every 2 or 3 range trips, unless I don't plan on shooting it for a while.

Carbon build up just like an AR. Usually match rifles that go many rounds without detail cleaning. Drills break up that glass like carbon built up really well. The piston tail is usually where the worst of it deposits over time.

I believe there’s a set of bits on the EE right now. If not Sadlak is where I sourced mine.




As I recall Springfield armory says nothing heavier then 180 that said 175s are quite popular.

The gas system shutting off the way it does, as does the shorter straighter op rod from my understanding help prevent this. Though one should be cognitive about to slow of powders, Reloader 16 comes to mind as the cutoff, battering the gun unnecessarily.

Any information to the contrary would be welcome.

175s  and M118LR were quite popular in the M1A.  The M1 Garand is picky on ammo if unmodified.

I know the guys on the M14forum love it and duplicates of it.

I thought about getting the drill bits for mine, but I've been able to scrub down the gas piston, and the tube, with just a toothbrush, and putting the piston in my ultrasonic cleaner.

I'd say those are for guys that clean their M1a infrequently, and where carbon has built up in the tube and gas piston, such that you need a bit to get it all off.

If you clean it after you shoot it, I don't see the need.