Yeah, definetly looks like a problem to me. Especially since the Bolter has a pretty economic full-auto and fire selectors come standart anyways, trivialising the ammo conservation argument a bit imo.
To me it´s a problem in terms "action durations" and giving each attack mode its niche.
Assuming you have a full action, what can you do with each attack mode?
Single shot: half action aim + shot; called shot
Semi-auto (bolter): +10 to hit, additional hits every 2 DoS (max 2 hits per burst). So here we have basically a single shot that may hit twice and includes a half action aim. Conclusion: half action aim + single shot is useless.
Full-auto (bolter): +20 to hit, additional hits every DoS (max 4 in total). A full action aim + single shot + up to 3 additional hits. Technically that´s 1 and a half action in one + possible additional hits. Conclusion: well, doesn´t need expaination...
So here´s are my thoughts about how to change that a bit. The goal is to give each firing mode its niche and purpose. Since I´m a bit new to the system and the thoughts come a bit spontaneous they may be flawed (like not considering advanced talents, forgetting about something,..). but here´s my go.
Single shots:
What´s their niche, what do we expect from them. Let´s go with accurate fire and called shots.
To emphasise that I went by improving the aim action and altering called shots a bit.
Aim action: grants +30 BS for full action and +20 for half. (<- does not appy to melee)
Called shot: is now a half-action but the difficulty is increased to very hard (-30). (<- does not apply to melee, adjust deadeye shot talent)
Conclusion: and aimed single shot is now the most accurate ranged attack, you can now aim and make a called shot in one turn.
Semiautomatic burst :
+10 BS, +20 at short range, (no bonus if the distance to the exceeds the weapons range); addional hit every DoS.
Our bread and Butter firing mode at higher ranges.
Full-auto burst:
+10 BS at short range (note: this is <75m with a Heavy Bolter); additional hit every DoS, additional hit every 2 DoS if the target is further away than short range.
So, what do you think? On a last note, I do not think that this is final or something but on any changes I´ll make, I´d emphasise on the same things.
So basically: Single shot -> accuracy, called shots. Semi burst -> standart bread´n butter firing mode, Full-auto -> short range mayhem.
If I do anything on the Heavy Bolter specifically, I´d probably emphasise on suppression.