Boron trifluoride proportional counters of calculable efficiency for neutron beam measurements are described. It has been found possible to obtain excellent pulse-size distributions with a filling of 60 cm of purified BF3 by ``soaking'' the counters for a long time (up to three months) with BF3 before putting in the final filling. These fillings are stable. Fillings of less than 5 cm BF3 made up to 50 cm with argon have also been studied. With these fillings the pulse-size distributions are very good for both beam and total irradiation conditions. A discussion of the counter efficiencies with particular reference to end correction is given.