As to the recoiling barrel of the Johnson it only moves back about 3/8" and is very solid thereafter. The user would have to prod a little farther.
In Canfield's book it is mentioned that MMJ saw a No4 bt on a promo trip to the UKicon and took the idea of the simple light "dagger bayonet" from that.
A previous Johnson bt design was fixed to the fore end and supported by a dovetail at the muzzle.