Today I attached a Harris BR bipod to my Howa 1500 in 30-06 and took it to the range.
Groups opened up from a regular 0,5" over sandbags to 1,5" and the POI shifted 1.5" up. (100 metres)

I found it much easier to be dead on to the target with the bipod attached but I cannot understand why groups open up, compared to shooting over sandbags. The barrel is fully free floated.

Any suggestions?

What surface was the bipod sitting on? If it was hard you'll notice that the rifle will jump up and your groups will be high.
I usually pad the area under the feet of the bipod with a soft cushion of some sort.

hth

Let me guess: synthetic stock, perhaps??

Some stocks (cheaper, injection molded synthetics are notorious) are too floppy in the forearm to properly support the pressure from shooting with a bipod. Barrels may feel like they are floated, but if the forearm is flexible enough it can flex and touch the barrel during firing.

- mike

Stock is a B&C FLBB stock; maybe not the best you can get but certainly no Tupperware junk.
I shot from a hardwood bench.
Ys, the bipod jumped off the bench every time, so I will try a layer of carpet or something like that under the bipod nxt time.

Yes they well affect accuracy. I use bipods a lot I have found that on my rifles that I have had to relive the barrel channels and with syt stock stiffen them with glass bedding.

I test them by putting the bipod on getting into a good prone position and have some one run a price of paper between the barrel and stock. I also put some pressureon the stock. If the barrel touchs I relive it until it no longer does. Once I get them properly bedded I find bipods do not affect them put greatly aids in makeing hits.