help needed to build a 300 whisper

[LouBoyd]

You need to make some choices giving some though about what you want from a "300 Whsper"

Bolt action (rem 700 or model 7) or AR-15? The obvious advantages of the AR15 are availability and rate of fire.
For low noise and ease of tuning ammo the bolt action has the advantage.

Suppressed or not? US Laws generally require use of a class 3 FFL dealer. $500-1500 for the suppressor depending on model, $200 tax, 6 month + wait.

Minimum barrel rifle (16") , NFA SBR, or pistol? ATF SBR registration is another $200 tax and wait plus cost of the short barrel).

Subsonic, supersonic, or both? A bolt action will handle all. . In an AR-15 it's difficult to get good performance with both. Solutions are an adjustable gas block or just have two uppers. The latter gives best performance.

Do you care about noise?

Only a subsonic bullet has low bullet noise ( 30+ decibels less than supersonic)
Only a suppressed rifle has low muzzle blast (30+ decibels less than with supersonic molecular propellant muzzle velocity)
Though 30 decibels is a reduction in noise energy of a factor of 1000, the range of human hearing from detection to pain is about 100 decibels. You'll still be able to hear a suppressed subsonic firearm at a considerable distance. A suppressed subsonic rifle usually does not require hearing protection.

Only a bolt action has low mechanical noise and no breach blast. That noise will be dominant with most suppressed subsonic semi-automatics. With a good suppressed bolt action you hear muzzle blast similar to an air rifle but the next loudest noise is from the bullet hitting the target.

If subsonic and quiet is the primary objective and cost is important there are other options. Long barrel rifles with large expansion ratios can also shoot bullets subsonic with low muzzle blast. the 300 Whisper/BLK is not a suitable cartridge. The advantage is no NFA involvement. But hat's a separate subject for another thread.