An issue that I know keeps submarine aficionados up at night is whether Russia will go to the trouble of fitting its 5 later model Borey 955A SSBNs, not with 16 SLBMs, but with 20. A myth persists on English language websites that from the 4th Borey/Borei (Project 955) SSBN onwards 20 SLBM silos will be fitted.
Russian Wikipedia, on Russian Subs, Better Than English
See English Wikipedia:
“Starting from the fourth hull, all submarines of the Borei class will have 20 missile tubes each, versus 16 for the first three boats.”
But see Russian Wikipedia, Project 955 Borey Submarines. Once in the Russian site wait 10 seconds for any "Translate" popup or right-click mouse to Translate. Note in the right sidebar “Armament” is:
“16 missiles of the D-30 complex  , SL -30 SS-NX-30 Bulava Number of missiles: 16 (project 955), 16 (project 955A) ”
So 20 Bulava SLBMs may be old news or still accurate.
One thought is that the US and Russia may see zero sum game threats (meaning "one person's gain is anothers loss") from North Korea (NK) and China building more nuclear warheads. Say 1,000 for China?
It is possible then the rising NK and Chinese capabilities would cause:
- the US to fit or retrofit 10m to 20m plugs (for 4 to 8 more SLBMs respectively) on to its new 16
SLBM future Columbia class SSBNs
- Russia might then retrofit 10m or 20m plugs onto its Boreys for the same purpose.
I think, instead the US would build additional Columbia and Russia additional Boreys all retaining 16 SLBMs. To fit or retrofit 10m or 20m plugs would mean many major adjustments would need to be made to the subs to maintain acceptable Columbia/Borey handling, steering and above all stealth.
Soumarsov and Pete